Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

[Kiyoshi Ueda]

Hi Christoph,

On 08/04/2010 03:51 AM +0900, Christoph Hellwig wrote:
> Adapt device-mapper to the new world order where even bio based devices
> get simple REQ_FLUSH requests for cache flushes, and need to submit
> them downwards for implementing barriers.
<snip>
> Index: linux-2.6/drivers/md/dm.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm.c	2010-08-03 20:26:49.676004139 +0200
> +++ linux-2.6/drivers/md/dm.c	2010-08-03 20:36:59.301005325 +0200
<snip>
> @@ -1573,7 +1565,7 @@ static int dm_prep_fn(struct request_que
>  	struct mapped_device *md = q->queuedata;
>  	struct request *clone;
>  
> -	if (unlikely(dm_rq_is_flush_request(rq)))
> +	if (rq->cmd_flags & REQ_FLUSH)
>  		return BLKPREP_OK;
>  
>  	if (unlikely(rq->special)) {
> @@ -1664,7 +1656,7 @@ static void dm_request_fn(struct request
>  		if (!rq)
>  			goto plug_and_out;
>  
> -		if (unlikely(dm_rq_is_flush_request(rq))) {
> +		if (rq->cmd_flags & REQ_FLUSH) {
>  			BUG_ON(md->flush_request);
>  			md->flush_request = rq;
>  			blk_start_request(rq);

Current request-based device-mapper's flush code depends on
the block-layer's barrier behavior which dispatches only one request
at a time when flush is needed.
In other words, current request-based device-mapper can't handle
other requests while a flush request is in progress.

I'll take a look how I can fix the request-based device-mapper to
cope with it.  I think it'll take time for carefull investigation.

Thanks,
Kiyoshi Ueda

Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

[Christoph Hellwig]

On Wed, Aug 04, 2010 at 01:57:37PM +0900, Kiyoshi Ueda wrote:
> > -		if (unlikely(dm_rq_is_flush_request(rq))) {
> > +		if (rq->cmd_flags & REQ_FLUSH) {
> >  			BUG_ON(md->flush_request);
> >  			md->flush_request = rq;
> >  			blk_start_request(rq);
> 
> Current request-based device-mapper's flush code depends on
> the block-layer's barrier behavior which dispatches only one request
> at a time when flush is needed.
> In other words, current request-based device-mapper can't handle
> other requests while a flush request is in progress.
> 
> I'll take a look how I can fix the request-based device-mapper to
> cope with it.  I think it'll take time for carefull investigation.

Given that request based device mapper doesn't even look at the
block numbers from what I can see just removing any special casing
for REQ_FLUSH should probably do it.

Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

[Jun'ichi Nomura]

Hi Christoph,

(08/04/10 17:54), Christoph Hellwig wrote:
> On Wed, Aug 04, 2010 at 01:57:37PM +0900, Kiyoshi Ueda wrote:
>>> -		if (unlikely(dm_rq_is_flush_request(rq))) {
>>> +		if (rq->cmd_flags & REQ_FLUSH) {
>>>  			BUG_ON(md->flush_request);
>>>  			md->flush_request = rq;
>>>  			blk_start_request(rq);
>>
>> Current request-based device-mapper's flush code depends on
>> the block-layer's barrier behavior which dispatches only one request
>> at a time when flush is needed.
>> In other words, current request-based device-mapper can't handle
>> other requests while a flush request is in progress.
>>
>> I'll take a look how I can fix the request-based device-mapper to
>> cope with it.  I think it'll take time for carefull investigation.
> 
> Given that request based device mapper doesn't even look at the
> block numbers from what I can see just removing any special casing
> for REQ_FLUSH should probably do it.

Special casing is necessary because device-mapper may have to
send multiple copies of REQ_FLUSH request to multiple
targets, while normal request is just sent to single target.

Thanks,
-- 
Jun'ichi Nomura, NEC Corporation

Re: [PATCH, RFC] relaxed barriers

[Tejun Heo]

Hello,

So, here's my shot at it.  After this patch, barrier no longer
dictates the ordering of other requests.  The block layer sequences
the barrier request without interfering with other requests (not even
elevator draining).  Multiple pending barriers are handled by saving
those in a separate queue and servicing them one by one.  Basically,
barrier sequences form a separate FIFO command stream independent of
other requests and all the ordering between the two streams is
filesystem's responsibility.

Ordered tag support is dropped as no one seems to be making any
meaningful use of it.  I'm fairly skeptical about its usefulness
anyway.  The only thing ordered tag saves is latencies between command
completions and issues in barrier sequences, which isn't much to begin
with and puts additional ordering restrictions compared to ordering in
software (ordered tag commands will unnecessary affect processing of
simple tag commands).

Lightly tested for all three BAR (!WC), FLUSH and FUA cases.  The
multiple pending barrier code path isn't tested yet.

Christoph, does this look like something the filesystems can use or
have I misunderstood something?

Thanks.

NOT_SIGNED_OFF_YET
---
 block/blk-barrier.c          |  253 +++++++++++++++----------------------------
 block/blk-core.c             |   31 ++---
 block/blk.h                  |    5
 block/elevator.c             |   80 +------------
 drivers/block/brd.c          |    2
 drivers/block/loop.c         |    2
 drivers/block/osdblk.c       |    2
 drivers/block/pktcdvd.c      |    1
 drivers/block/ps3disk.c      |    3
 drivers/block/virtio_blk.c   |    4
 drivers/block/xen-blkfront.c |    2
 drivers/ide/ide-disk.c       |    4
 drivers/md/dm.c              |    3
 drivers/mmc/card/queue.c     |    2
 drivers/s390/block/dasd.c    |    2
 drivers/scsi/sd.c            |    8 -
 include/linux/blkdev.h       |   59 +++-------
 include/linux/elevator.h     |    6 -
 18 files changed, 154 insertions(+), 315 deletions(-)

Index: work/block/blk-barrier.c
===================================================================
--- work.orig/block/blk-barrier.c
+++ work/block/blk-barrier.c
@@ -9,6 +9,8 @@

 #include "blk.h"

+static struct request *queue_next_ordseq(struct request_queue *q);
+
 /**
  * blk_queue_ordered - does this queue support ordered writes
  * @q:        the request queue
@@ -31,13 +33,8 @@ int blk_queue_ordered(struct request_que
 		return -EINVAL;
 	}

-	if (ordered != QUEUE_ORDERED_NONE &&
-	    ordered != QUEUE_ORDERED_DRAIN &&
-	    ordered != QUEUE_ORDERED_DRAIN_FLUSH &&
-	    ordered != QUEUE_ORDERED_DRAIN_FUA &&
-	    ordered != QUEUE_ORDERED_TAG &&
-	    ordered != QUEUE_ORDERED_TAG_FLUSH &&
-	    ordered != QUEUE_ORDERED_TAG_FUA) {
+	if (ordered != QUEUE_ORDERED_NONE && ordered != QUEUE_ORDERED_BAR &&
+	    ordered != QUEUE_ORDERED_FLUSH && ordered != QUEUE_ORDERED_FUA) {
 		printk(KERN_ERR "blk_queue_ordered: bad value %d\n", ordered);
 		return -EINVAL;
 	}
@@ -60,38 +57,10 @@ unsigned blk_ordered_cur_seq(struct requ
 	return 1 << ffz(q->ordseq);
 }

-unsigned blk_ordered_req_seq(struct request *rq)
+static struct request *blk_ordered_complete_seq(struct request_queue *q,
+						unsigned seq, int error)
 {
-	struct request_queue *q = rq->q;
-
-	BUG_ON(q->ordseq == 0);
-
-	if (rq == &q->pre_flush_rq)
-		return QUEUE_ORDSEQ_PREFLUSH;
-	if (rq == &q->bar_rq)
-		return QUEUE_ORDSEQ_BAR;
-	if (rq == &q->post_flush_rq)
-		return QUEUE_ORDSEQ_POSTFLUSH;
-
-	/*
-	 * !fs requests don't need to follow barrier ordering.  Always
-	 * put them at the front.  This fixes the following deadlock.
-	 *
-	 * http://thread.gmane.org/gmane.linux.kernel/537473
-	 */
-	if (!blk_fs_request(rq))
-		return QUEUE_ORDSEQ_DRAIN;
-
-	if ((rq->cmd_flags & REQ_ORDERED_COLOR) ==
-	    (q->orig_bar_rq->cmd_flags & REQ_ORDERED_COLOR))
-		return QUEUE_ORDSEQ_DRAIN;
-	else
-		return QUEUE_ORDSEQ_DONE;
-}
-
-bool blk_ordered_complete_seq(struct request_queue *q, unsigned seq, int error)
-{
-	struct request *rq;
+	struct request *rq = NULL;

 	if (error && !q->orderr)
 		q->orderr = error;
@@ -99,16 +68,22 @@ bool blk_ordered_complete_seq(struct req
 	BUG_ON(q->ordseq & seq);
 	q->ordseq |= seq;

-	if (blk_ordered_cur_seq(q) != QUEUE_ORDSEQ_DONE)
-		return false;
-
-	/*
-	 * Okay, sequence complete.
-	 */
-	q->ordseq = 0;
-	rq = q->orig_bar_rq;
-	__blk_end_request_all(rq, q->orderr);
-	return true;
+	if (blk_ordered_cur_seq(q) != QUEUE_ORDSEQ_DONE) {
+		/* not complete yet, queue the next ordered sequence */
+		rq = queue_next_ordseq(q);
+	} else {
+		/* complete this barrier request */
+		__blk_end_request_all(q->orig_bar_rq, q->orderr);
+		q->orig_bar_rq = NULL;
+		q->ordseq = 0;
+
+		/* dispatch the next barrier if there's one */
+		if (!list_empty(&q->pending_barriers)) {
+			rq = list_entry_rq(q->pending_barriers.next);
+			list_move(&rq->queuelist, &q->queue_head);
+		}
+	}
+	return rq;
 }

 static void pre_flush_end_io(struct request *rq, int error)
@@ -129,21 +104,10 @@ static void post_flush_end_io(struct req
 	blk_ordered_complete_seq(rq->q, QUEUE_ORDSEQ_POSTFLUSH, error);
 }

-static void queue_flush(struct request_queue *q, unsigned which)
+static void queue_flush(struct request_queue *q, struct request *rq,
+			rq_end_io_fn *end_io)
 {
-	struct request *rq;
-	rq_end_io_fn *end_io;
-
-	if (which == QUEUE_ORDERED_DO_PREFLUSH) {
-		rq = &q->pre_flush_rq;
-		end_io = pre_flush_end_io;
-	} else {
-		rq = &q->post_flush_rq;
-		end_io = post_flush_end_io;
-	}
-
 	blk_rq_init(q, rq);
-	rq->cmd_flags = REQ_HARDBARRIER;
 	rq->rq_disk = q->bar_rq.rq_disk;
 	rq->end_io = end_io;
 	q->prepare_flush_fn(q, rq);
@@ -151,130 +115,93 @@ static void queue_flush(struct request_q
 	elv_insert(q, rq, ELEVATOR_INSERT_FRONT);
 }

-static inline bool start_ordered(struct request_queue *q, struct request **rqp)
+static struct request *queue_next_ordseq(struct request_queue *q)
 {
-	struct request *rq = *rqp;
-	unsigned skip = 0;
-
-	q->orderr = 0;
-	q->ordered = q->next_ordered;
-	q->ordseq |= QUEUE_ORDSEQ_STARTED;
-
-	/*
-	 * For an empty barrier, there's no actual BAR request, which
-	 * in turn makes POSTFLUSH unnecessary.  Mask them off.
-	 */
-	if (!blk_rq_sectors(rq)) {
-		q->ordered &= ~(QUEUE_ORDERED_DO_BAR |
-				QUEUE_ORDERED_DO_POSTFLUSH);
-		/*
-		 * Empty barrier on a write-through device w/ ordered
-		 * tag has no command to issue and without any command
-		 * to issue, ordering by tag can't be used.  Drain
-		 * instead.
-		 */
-		if ((q->ordered & QUEUE_ORDERED_BY_TAG) &&
-		    !(q->ordered & QUEUE_ORDERED_DO_PREFLUSH))
-			q->ordered &= ~QUEUE_ORDERED_BY_TAG;
-	}
-
-	/* stash away the original request */
-	blk_dequeue_request(rq);
-	q->orig_bar_rq = rq;
-	rq = NULL;
-
-	/*
-	 * Queue ordered sequence.  As we stack them at the head, we
-	 * need to queue in reverse order.  Note that we rely on that
-	 * no fs request uses ELEVATOR_INSERT_FRONT and thus no fs
-	 * request gets inbetween ordered sequence.
-	 */
-	if (q->ordered & QUEUE_ORDERED_DO_POSTFLUSH) {
-		queue_flush(q, QUEUE_ORDERED_DO_POSTFLUSH);
-		rq = &q->post_flush_rq;
-	} else
-		skip |= QUEUE_ORDSEQ_POSTFLUSH;
+	struct request *rq = &q->bar_rq;

-	if (q->ordered & QUEUE_ORDERED_DO_BAR) {
-		rq = &q->bar_rq;
+	switch (blk_ordered_cur_seq(q)) {
+	case QUEUE_ORDSEQ_PREFLUSH:
+		queue_flush(q, rq, pre_flush_end_io);
+		break;

+	case QUEUE_ORDSEQ_BAR:
 		/* initialize proxy request and queue it */
 		blk_rq_init(q, rq);
-		if (bio_data_dir(q->orig_bar_rq->bio) == WRITE)
-			rq->cmd_flags |= REQ_RW;
+		init_request_from_bio(rq, q->orig_bar_rq->bio);
+		rq->cmd_flags &= ~REQ_HARDBARRIER;
 		if (q->ordered & QUEUE_ORDERED_DO_FUA)
 			rq->cmd_flags |= REQ_FUA;
-		init_request_from_bio(rq, q->orig_bar_rq->bio);
 		rq->end_io = bar_end_io;

 		elv_insert(q, rq, ELEVATOR_INSERT_FRONT);
-	} else
-		skip |= QUEUE_ORDSEQ_BAR;
+		break;

-	if (q->ordered & QUEUE_ORDERED_DO_PREFLUSH) {
-		queue_flush(q, QUEUE_ORDERED_DO_PREFLUSH);
-		rq = &q->pre_flush_rq;
-	} else
-		skip |= QUEUE_ORDSEQ_PREFLUSH;
+	case QUEUE_ORDSEQ_POSTFLUSH:
+		queue_flush(q, rq, post_flush_end_io);
+		break;

-	if (!(q->ordered & QUEUE_ORDERED_BY_TAG) && queue_in_flight(q))
-		rq = NULL;
-	else
-		skip |= QUEUE_ORDSEQ_DRAIN;
-
-	*rqp = rq;
-
-	/*
-	 * Complete skipped sequences.  If whole sequence is complete,
-	 * return false to tell elevator that this request is gone.
-	 */
-	return !blk_ordered_complete_seq(q, skip, 0);
+	default:
+		BUG();
+	}
+	return rq;
 }

-bool blk_do_ordered(struct request_queue *q, struct request **rqp)
+struct request *blk_do_ordered(struct request_queue *q, struct request *rq)
 {
-	struct request *rq = *rqp;
-	const int is_barrier = blk_fs_request(rq) && blk_barrier_rq(rq);
+	unsigned skip = 0;

-	if (!q->ordseq) {
-		if (!is_barrier)
-			return true;
-
-		if (q->next_ordered != QUEUE_ORDERED_NONE)
-			return start_ordered(q, rqp);
-		else {
-			/*
-			 * Queue ordering not supported.  Terminate
-			 * with prejudice.
-			 */
-			blk_dequeue_request(rq);
-			__blk_end_request_all(rq, -EOPNOTSUPP);
-			*rqp = NULL;
-			return false;
-		}
+	if (!blk_barrier_rq(rq))
+		return rq;
+
+	if (q->ordseq) {
+		/*
+		 * Barrier is already in progress and they can't be
+		 * processed in parallel.  Queue for later processing.
+		 */
+		list_move_tail(&rq->queuelist, &q->pending_barriers);
+		return NULL;
+	}
+
+	if (unlikely(q->next_ordered == QUEUE_ORDERED_NONE)) {
+		/*
+		 * Queue ordering not supported.  Terminate
+		 * with prejudice.
+		 */
+		blk_dequeue_request(rq);
+		__blk_end_request_all(rq, -EOPNOTSUPP);
+		return NULL;
 	}

 	/*
-	 * Ordered sequence in progress
+	 * Start a new ordered sequence
 	 */
+	q->orderr = 0;
+	q->ordered = q->next_ordered;
+	q->ordseq |= QUEUE_ORDSEQ_STARTED;

-	/* Special requests are not subject to ordering rules. */
-	if (!blk_fs_request(rq) &&
-	    rq != &q->pre_flush_rq && rq != &q->post_flush_rq)
-		return true;
-
-	if (q->ordered & QUEUE_ORDERED_BY_TAG) {
-		/* Ordered by tag.  Blocking the next barrier is enough. */
-		if (is_barrier && rq != &q->bar_rq)
-			*rqp = NULL;
-	} else {
-		/* Ordered by draining.  Wait for turn. */
-		WARN_ON(blk_ordered_req_seq(rq) < blk_ordered_cur_seq(q));
-		if (blk_ordered_req_seq(rq) > blk_ordered_cur_seq(q))
-			*rqp = NULL;
-	}
+	/*
+	 * For an empty barrier, there's no actual BAR request, which
+	 * in turn makes POSTFLUSH unnecessary.  Mask them off.
+	 */
+	if (!blk_rq_sectors(rq))
+		q->ordered &= ~(QUEUE_ORDERED_DO_BAR |
+				QUEUE_ORDERED_DO_POSTFLUSH);
+
+	/* stash away the original request */
+	blk_dequeue_request(rq);
+	q->orig_bar_rq = rq;
+
+	if (!(q->ordered & QUEUE_ORDERED_DO_PREFLUSH))
+		skip |= QUEUE_ORDSEQ_PREFLUSH;
+
+	if (!(q->ordered & QUEUE_ORDERED_DO_BAR))
+		skip |= QUEUE_ORDSEQ_BAR;
+
+	if (!(q->ordered & QUEUE_ORDERED_DO_POSTFLUSH))
+		skip |= QUEUE_ORDSEQ_POSTFLUSH;

-	return true;
+	/* complete skipped sequences and return the first sequence */
+	return blk_ordered_complete_seq(q, skip, 0);
 }

 static void bio_end_empty_barrier(struct bio *bio, int err)
Index: work/include/linux/blkdev.h
===================================================================
--- work.orig/include/linux/blkdev.h
+++ work/include/linux/blkdev.h
@@ -106,7 +106,6 @@ enum rq_flag_bits {
 	__REQ_FAILED,		/* set if the request failed */
 	__REQ_QUIET,		/* don't worry about errors */
 	__REQ_PREEMPT,		/* set for "ide_preempt" requests */
-	__REQ_ORDERED_COLOR,	/* is before or after barrier */
 	__REQ_RW_SYNC,		/* request is sync (sync write or read) */
 	__REQ_ALLOCED,		/* request came from our alloc pool */
 	__REQ_RW_META,		/* metadata io request */
@@ -135,7 +134,6 @@ enum rq_flag_bits {
 #define REQ_FAILED	(1 << __REQ_FAILED)
 #define REQ_QUIET	(1 << __REQ_QUIET)
 #define REQ_PREEMPT	(1 << __REQ_PREEMPT)
-#define REQ_ORDERED_COLOR	(1 << __REQ_ORDERED_COLOR)
 #define REQ_RW_SYNC	(1 << __REQ_RW_SYNC)
 #define REQ_ALLOCED	(1 << __REQ_ALLOCED)
 #define REQ_RW_META	(1 << __REQ_RW_META)
@@ -437,9 +435,10 @@ struct request_queue
 	 * reserved for flush operations
 	 */
 	unsigned int		ordered, next_ordered, ordseq;
-	int			orderr, ordcolor;
-	struct request		pre_flush_rq, bar_rq, post_flush_rq;
-	struct request		*orig_bar_rq;
+	int			orderr;
+	struct request		bar_rq;
+	struct request          *orig_bar_rq;
+	struct list_head	pending_barriers;

 	struct mutex		sysfs_lock;

@@ -543,47 +542,33 @@ enum {
 	 * Hardbarrier is supported with one of the following methods.
 	 *
 	 * NONE		: hardbarrier unsupported
-	 * DRAIN	: ordering by draining is enough
-	 * DRAIN_FLUSH	: ordering by draining w/ pre and post flushes
-	 * DRAIN_FUA	: ordering by draining w/ pre flush and FUA write
-	 * TAG		: ordering by tag is enough
-	 * TAG_FLUSH	: ordering by tag w/ pre and post flushes
-	 * TAG_FUA	: ordering by tag w/ pre flush and FUA write
-	 */
-	QUEUE_ORDERED_BY_TAG		= 0x02,
-	QUEUE_ORDERED_DO_PREFLUSH	= 0x10,
-	QUEUE_ORDERED_DO_BAR		= 0x20,
-	QUEUE_ORDERED_DO_POSTFLUSH	= 0x40,
-	QUEUE_ORDERED_DO_FUA		= 0x80,
+	 * BAR		: writing out barrier is enough
+	 * FLUSH	: barrier and surrounding pre and post flushes
+	 * FUA		: FUA barrier w/ pre flush
+	 */
+	QUEUE_ORDERED_DO_PREFLUSH	= 1 << 0,
+	QUEUE_ORDERED_DO_BAR		= 1 << 1,
+	QUEUE_ORDERED_DO_POSTFLUSH	= 1 << 2,
+	QUEUE_ORDERED_DO_FUA		= 1 << 3,

-	QUEUE_ORDERED_NONE		= 0x00,
+	QUEUE_ORDERED_NONE		= 0,

-	QUEUE_ORDERED_DRAIN		= QUEUE_ORDERED_DO_BAR,
-	QUEUE_ORDERED_DRAIN_FLUSH	= QUEUE_ORDERED_DRAIN |
+	QUEUE_ORDERED_BAR		= QUEUE_ORDERED_DO_BAR,
+	QUEUE_ORDERED_FLUSH		= QUEUE_ORDERED_DO_BAR |
 					  QUEUE_ORDERED_DO_PREFLUSH |
 					  QUEUE_ORDERED_DO_POSTFLUSH,
-	QUEUE_ORDERED_DRAIN_FUA		= QUEUE_ORDERED_DRAIN |
-					  QUEUE_ORDERED_DO_PREFLUSH |
-					  QUEUE_ORDERED_DO_FUA,
-
-	QUEUE_ORDERED_TAG		= QUEUE_ORDERED_BY_TAG |
-					  QUEUE_ORDERED_DO_BAR,
-	QUEUE_ORDERED_TAG_FLUSH		= QUEUE_ORDERED_TAG |
-					  QUEUE_ORDERED_DO_PREFLUSH |
-					  QUEUE_ORDERED_DO_POSTFLUSH,
-	QUEUE_ORDERED_TAG_FUA		= QUEUE_ORDERED_TAG |
+	QUEUE_ORDERED_FUA		= QUEUE_ORDERED_DO_BAR |
 					  QUEUE_ORDERED_DO_PREFLUSH |
 					  QUEUE_ORDERED_DO_FUA,

 	/*
 	 * Ordered operation sequence
 	 */
-	QUEUE_ORDSEQ_STARTED	= 0x01,	/* flushing in progress */
-	QUEUE_ORDSEQ_DRAIN	= 0x02,	/* waiting for the queue to be drained */
-	QUEUE_ORDSEQ_PREFLUSH	= 0x04,	/* pre-flushing in progress */
-	QUEUE_ORDSEQ_BAR	= 0x08,	/* original barrier req in progress */
-	QUEUE_ORDSEQ_POSTFLUSH	= 0x10,	/* post-flushing in progress */
-	QUEUE_ORDSEQ_DONE	= 0x20,
+	QUEUE_ORDSEQ_STARTED	= (1 << 0), /* flushing in progress */
+	QUEUE_ORDSEQ_PREFLUSH	= (1 << 1), /* pre-flushing in progress */
+	QUEUE_ORDSEQ_BAR	= (1 << 2), /* barrier write in progress */
+	QUEUE_ORDSEQ_POSTFLUSH	= (1 << 3), /* post-flushing in progress */
+	QUEUE_ORDSEQ_DONE	= (1 << 4),
 };

 #define blk_queue_plugged(q)	test_bit(QUEUE_FLAG_PLUGGED, &(q)->queue_flags)
@@ -965,10 +950,8 @@ extern void blk_queue_rq_timed_out(struc
 extern void blk_queue_rq_timeout(struct request_queue *, unsigned int);
 extern struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev);
 extern int blk_queue_ordered(struct request_queue *, unsigned, prepare_flush_fn *);
-extern bool blk_do_ordered(struct request_queue *, struct request **);
 extern unsigned blk_ordered_cur_seq(struct request_queue *);
 extern unsigned blk_ordered_req_seq(struct request *);
-extern bool blk_ordered_complete_seq(struct request_queue *, unsigned, int);

 extern int blk_rq_map_sg(struct request_queue *, struct request *, struct scatterlist *);
 extern void blk_dump_rq_flags(struct request *, char *);
Index: work/drivers/block/brd.c
===================================================================
--- work.orig/drivers/block/brd.c
+++ work/drivers/block/brd.c
@@ -479,7 +479,7 @@ static struct brd_device *brd_alloc(int
 	if (!brd->brd_queue)
 		goto out_free_dev;
 	blk_queue_make_request(brd->brd_queue, brd_make_request);
-	blk_queue_ordered(brd->brd_queue, QUEUE_ORDERED_TAG, NULL);
+	blk_queue_ordered(brd->brd_queue, QUEUE_ORDERED_BAR, NULL);
 	blk_queue_max_hw_sectors(brd->brd_queue, 1024);
 	blk_queue_bounce_limit(brd->brd_queue, BLK_BOUNCE_ANY);

Index: work/drivers/block/virtio_blk.c
===================================================================
--- work.orig/drivers/block/virtio_blk.c
+++ work/drivers/block/virtio_blk.c
@@ -368,10 +368,10 @@ static int __devinit virtblk_probe(struc

 	/* If barriers are supported, tell block layer that queue is ordered */
 	if (virtio_has_feature(vdev, VIRTIO_BLK_F_FLUSH))
-		blk_queue_ordered(q, QUEUE_ORDERED_DRAIN_FLUSH,
+		blk_queue_ordered(q, QUEUE_ORDERED_FLUSH,
 				  virtblk_prepare_flush);
 	else if (virtio_has_feature(vdev, VIRTIO_BLK_F_BARRIER))
-		blk_queue_ordered(q, QUEUE_ORDERED_TAG, NULL);
+		blk_queue_ordered(q, QUEUE_ORDERED_BAR, NULL);

 	/* If disk is read-only in the host, the guest should obey */
 	if (virtio_has_feature(vdev, VIRTIO_BLK_F_RO))
Index: work/drivers/scsi/sd.c
===================================================================
--- work.orig/drivers/scsi/sd.c
+++ work/drivers/scsi/sd.c
@@ -2103,15 +2103,13 @@ static int sd_revalidate_disk(struct gen

 	/*
 	 * We now have all cache related info, determine how we deal
-	 * with ordered requests.  Note that as the current SCSI
-	 * dispatch function can alter request order, we cannot use
-	 * QUEUE_ORDERED_TAG_* even when ordered tag is supported.
+	 * with ordered requests.
 	 */
 	if (sdkp->WCE)
 		ordered = sdkp->DPOFUA
-			? QUEUE_ORDERED_DRAIN_FUA : QUEUE_ORDERED_DRAIN_FLUSH;
+			? QUEUE_ORDERED_FUA : QUEUE_ORDERED_FLUSH;
 	else
-		ordered = QUEUE_ORDERED_DRAIN;
+		ordered = QUEUE_ORDERED_BAR;

 	blk_queue_ordered(sdkp->disk->queue, ordered, sd_prepare_flush);

Index: work/block/blk-core.c
===================================================================
--- work.orig/block/blk-core.c
+++ work/block/blk-core.c
@@ -520,6 +520,7 @@ struct request_queue *blk_alloc_queue_no
 	init_timer(&q->unplug_timer);
 	setup_timer(&q->timeout, blk_rq_timed_out_timer, (unsigned long) q);
 	INIT_LIST_HEAD(&q->timeout_list);
+	INIT_LIST_HEAD(&q->pending_barriers);
 	INIT_WORK(&q->unplug_work, blk_unplug_work);

 	kobject_init(&q->kobj, &blk_queue_ktype);
@@ -1036,22 +1037,6 @@ void blk_insert_request(struct request_q
 }
 EXPORT_SYMBOL(blk_insert_request);

-/*
- * add-request adds a request to the linked list.
- * queue lock is held and interrupts disabled, as we muck with the
- * request queue list.
- */
-static inline void add_request(struct request_queue *q, struct request *req)
-{
-	drive_stat_acct(req, 1);
-
-	/*
-	 * elevator indicated where it wants this request to be
-	 * inserted at elevator_merge time
-	 */
-	__elv_add_request(q, req, ELEVATOR_INSERT_SORT, 0);
-}
-
 static void part_round_stats_single(int cpu, struct hd_struct *part,
 				    unsigned long now)
 {
@@ -1184,6 +1169,7 @@ static int __make_request(struct request
 	const bool sync = bio_rw_flagged(bio, BIO_RW_SYNCIO);
 	const bool unplug = bio_rw_flagged(bio, BIO_RW_UNPLUG);
 	const unsigned int ff = bio->bi_rw & REQ_FAILFAST_MASK;
+	int where = ELEVATOR_INSERT_SORT;
 	int rw_flags;

 	if (bio_rw_flagged(bio, BIO_RW_BARRIER) &&
@@ -1191,6 +1177,7 @@ static int __make_request(struct request
 		bio_endio(bio, -EOPNOTSUPP);
 		return 0;
 	}
+
 	/*
 	 * low level driver can indicate that it wants pages above a
 	 * certain limit bounced to low memory (ie for highmem, or even
@@ -1200,7 +1187,12 @@ static int __make_request(struct request

 	spin_lock_irq(q->queue_lock);

-	if (unlikely(bio_rw_flagged(bio, BIO_RW_BARRIER)) || elv_queue_empty(q))
+	if (bio_rw_flagged(bio, BIO_RW_BARRIER)) {
+		where = ELEVATOR_INSERT_ORDERED;
+		goto get_rq;
+	}
+
+	if (elv_queue_empty(q))
 		goto get_rq;

 	el_ret = elv_merge(q, &req, bio);
@@ -1297,7 +1289,10 @@ get_rq:
 		req->cpu = blk_cpu_to_group(smp_processor_id());
 	if (queue_should_plug(q) && elv_queue_empty(q))
 		blk_plug_device(q);
-	add_request(q, req);
+
+	/* insert the request into the elevator */
+	drive_stat_acct(req, 1);
+	__elv_add_request(q, req, where, 0);
 out:
 	if (unplug || !queue_should_plug(q))
 		__generic_unplug_device(q);
Index: work/block/elevator.c
===================================================================
--- work.orig/block/elevator.c
+++ work/block/elevator.c
@@ -564,7 +564,7 @@ void elv_requeue_request(struct request_

 	rq->cmd_flags &= ~REQ_STARTED;

-	elv_insert(q, rq, ELEVATOR_INSERT_REQUEUE);
+	elv_insert(q, rq, ELEVATOR_INSERT_FRONT);
 }

 void elv_drain_elevator(struct request_queue *q)
@@ -611,8 +611,6 @@ void elv_quiesce_end(struct request_queu

 void elv_insert(struct request_queue *q, struct request *rq, int where)
 {
-	struct list_head *pos;
-	unsigned ordseq;
 	int unplug_it = 1;

 	trace_block_rq_insert(q, rq);
@@ -622,10 +620,14 @@ void elv_insert(struct request_queue *q,
 	switch (where) {
 	case ELEVATOR_INSERT_FRONT:
 		rq->cmd_flags |= REQ_SOFTBARRIER;
-
 		list_add(&rq->queuelist, &q->queue_head);
 		break;

+	case ELEVATOR_INSERT_ORDERED:
+		rq->cmd_flags |= REQ_SOFTBARRIER;
+		list_add_tail(&rq->queuelist, &q->queue_head);
+		break;
+
 	case ELEVATOR_INSERT_BACK:
 		rq->cmd_flags |= REQ_SOFTBARRIER;
 		elv_drain_elevator(q);
@@ -661,36 +663,6 @@ void elv_insert(struct request_queue *q,
 		q->elevator->ops->elevator_add_req_fn(q, rq);
 		break;

-	case ELEVATOR_INSERT_REQUEUE:
-		/*
-		 * If ordered flush isn't in progress, we do front
-		 * insertion; otherwise, requests should be requeued
-		 * in ordseq order.
-		 */
-		rq->cmd_flags |= REQ_SOFTBARRIER;
-
-		/*
-		 * Most requeues happen because of a busy condition,
-		 * don't force unplug of the queue for that case.
-		 */
-		unplug_it = 0;
-
-		if (q->ordseq == 0) {
-			list_add(&rq->queuelist, &q->queue_head);
-			break;
-		}
-
-		ordseq = blk_ordered_req_seq(rq);
-
-		list_for_each(pos, &q->queue_head) {
-			struct request *pos_rq = list_entry_rq(pos);
-			if (ordseq <= blk_ordered_req_seq(pos_rq))
-				break;
-		}
-
-		list_add_tail(&rq->queuelist, pos);
-		break;
-
 	default:
 		printk(KERN_ERR "%s: bad insertion point %d\n",
 		       __func__, where);
@@ -709,32 +681,14 @@ void elv_insert(struct request_queue *q,
 void __elv_add_request(struct request_queue *q, struct request *rq, int where,
 		       int plug)
 {
-	if (q->ordcolor)
-		rq->cmd_flags |= REQ_ORDERED_COLOR;
-
 	if (rq->cmd_flags & (REQ_SOFTBARRIER | REQ_HARDBARRIER)) {
-		/*
-		 * toggle ordered color
-		 */
-		if (blk_barrier_rq(rq))
-			q->ordcolor ^= 1;
-
-		/*
-		 * barriers implicitly indicate back insertion
-		 */
-		if (where == ELEVATOR_INSERT_SORT)
-			where = ELEVATOR_INSERT_BACK;
-
-		/*
-		 * this request is scheduling boundary, update
-		 * end_sector
-		 */
+		/* barriers are scheduling boundary, update end_sector */
 		if (blk_fs_request(rq) || blk_discard_rq(rq)) {
 			q->end_sector = rq_end_sector(rq);
 			q->boundary_rq = rq;
 		}
 	} else if (!(rq->cmd_flags & REQ_ELVPRIV) &&
-		    where == ELEVATOR_INSERT_SORT)
+		   where == ELEVATOR_INSERT_SORT)
 		where = ELEVATOR_INSERT_BACK;

 	if (plug)
@@ -846,24 +800,6 @@ void elv_completed_request(struct reques
 		if (blk_sorted_rq(rq) && e->ops->elevator_completed_req_fn)
 			e->ops->elevator_completed_req_fn(q, rq);
 	}
-
-	/*
-	 * Check if the queue is waiting for fs requests to be
-	 * drained for flush sequence.
-	 */
-	if (unlikely(q->ordseq)) {
-		struct request *next = NULL;
-
-		if (!list_empty(&q->queue_head))
-			next = list_entry_rq(q->queue_head.next);
-
-		if (!queue_in_flight(q) &&
-		    blk_ordered_cur_seq(q) == QUEUE_ORDSEQ_DRAIN &&
-		    (!next || blk_ordered_req_seq(next) > QUEUE_ORDSEQ_DRAIN)) {
-			blk_ordered_complete_seq(q, QUEUE_ORDSEQ_DRAIN, 0);
-			__blk_run_queue(q);
-		}
-	}
 }

 #define to_elv(atr) container_of((atr), struct elv_fs_entry, attr)
Index: work/block/blk.h
===================================================================
--- work.orig/block/blk.h
+++ work/block/blk.h
@@ -51,6 +51,8 @@ static inline void blk_clear_rq_complete
  */
 #define ELV_ON_HASH(rq)		(!hlist_unhashed(&(rq)->hash))

+struct request *blk_do_ordered(struct request_queue *q, struct request *rq);
+
 static inline struct request *__elv_next_request(struct request_queue *q)
 {
 	struct request *rq;
@@ -58,7 +60,8 @@ static inline struct request *__elv_next
 	while (1) {
 		while (!list_empty(&q->queue_head)) {
 			rq = list_entry_rq(q->queue_head.next);
-			if (blk_do_ordered(q, &rq))
+			rq = blk_do_ordered(q, rq);
+			if (rq)
 				return rq;
 		}

Index: work/drivers/block/loop.c
===================================================================
--- work.orig/drivers/block/loop.c
+++ work/drivers/block/loop.c
@@ -831,7 +831,7 @@ static int loop_set_fd(struct loop_devic
 	lo->lo_queue->unplug_fn = loop_unplug;

 	if (!(lo_flags & LO_FLAGS_READ_ONLY) && file->f_op->fsync)
-		blk_queue_ordered(lo->lo_queue, QUEUE_ORDERED_DRAIN, NULL);
+		blk_queue_ordered(lo->lo_queue, QUEUE_ORDERED_BAR, NULL);

 	set_capacity(lo->lo_disk, size);
 	bd_set_size(bdev, size << 9);
Index: work/drivers/block/osdblk.c
===================================================================
--- work.orig/drivers/block/osdblk.c
+++ work/drivers/block/osdblk.c
@@ -446,7 +446,7 @@ static int osdblk_init_disk(struct osdbl
 	blk_queue_stack_limits(q, osd_request_queue(osdev->osd));

 	blk_queue_prep_rq(q, blk_queue_start_tag);
-	blk_queue_ordered(q, QUEUE_ORDERED_DRAIN_FLUSH, osdblk_prepare_flush);
+	blk_queue_ordered(q, QUEUE_ORDERED_FLUSH, osdblk_prepare_flush);

 	disk->queue = q;

Index: work/drivers/block/ps3disk.c
===================================================================
--- work.orig/drivers/block/ps3disk.c
+++ work/drivers/block/ps3disk.c
@@ -480,8 +480,7 @@ static int __devinit ps3disk_probe(struc
 	blk_queue_dma_alignment(queue, dev->blk_size-1);
 	blk_queue_logical_block_size(queue, dev->blk_size);

-	blk_queue_ordered(queue, QUEUE_ORDERED_DRAIN_FLUSH,
-			  ps3disk_prepare_flush);
+	blk_queue_ordered(queue, QUEUE_ORDERED_FLUSH, ps3disk_prepare_flush);

 	blk_queue_max_segments(queue, -1);
 	blk_queue_max_segment_size(queue, dev->bounce_size);
Index: work/drivers/block/xen-blkfront.c
===================================================================
--- work.orig/drivers/block/xen-blkfront.c
+++ work/drivers/block/xen-blkfront.c
@@ -373,7 +373,7 @@ static int xlvbd_barrier(struct blkfront
 	int err;

 	err = blk_queue_ordered(info->rq,
-				info->feature_barrier ? QUEUE_ORDERED_DRAIN : QUEUE_ORDERED_NONE,
+				info->feature_barrier ? QUEUE_ORDERED_BAR : QUEUE_ORDERED_NONE,
 				NULL);

 	if (err)
Index: work/drivers/ide/ide-disk.c
===================================================================
--- work.orig/drivers/ide/ide-disk.c
+++ work/drivers/ide/ide-disk.c
@@ -537,11 +537,11 @@ static void update_ordered(ide_drive_t *
 		       drive->name, barrier ? "" : "not ");

 		if (barrier) {
-			ordered = QUEUE_ORDERED_DRAIN_FLUSH;
+			ordered = QUEUE_ORDERED_FLUSH;
 			prep_fn = idedisk_prepare_flush;
 		}
 	} else
-		ordered = QUEUE_ORDERED_DRAIN;
+		ordered = QUEUE_ORDERED_BAR;

 	blk_queue_ordered(drive->queue, ordered, prep_fn);
 }
Index: work/drivers/md/dm.c
===================================================================
--- work.orig/drivers/md/dm.c
+++ work/drivers/md/dm.c
@@ -1912,8 +1912,7 @@ static struct mapped_device *alloc_dev(i
 	blk_queue_softirq_done(md->queue, dm_softirq_done);
 	blk_queue_prep_rq(md->queue, dm_prep_fn);
 	blk_queue_lld_busy(md->queue, dm_lld_busy);
-	blk_queue_ordered(md->queue, QUEUE_ORDERED_DRAIN_FLUSH,
-			  dm_rq_prepare_flush);
+	blk_queue_ordered(md->queue, QUEUE_ORDERED_FLUSH, dm_rq_prepare_flush);

 	md->disk = alloc_disk(1);
 	if (!md->disk)
Index: work/drivers/mmc/card/queue.c
===================================================================
--- work.orig/drivers/mmc/card/queue.c
+++ work/drivers/mmc/card/queue.c
@@ -128,7 +128,7 @@ int mmc_init_queue(struct mmc_queue *mq,
 	mq->req = NULL;

 	blk_queue_prep_rq(mq->queue, mmc_prep_request);
-	blk_queue_ordered(mq->queue, QUEUE_ORDERED_DRAIN, NULL);
+	blk_queue_ordered(mq->queue, QUEUE_ORDERED_BAR, NULL);
 	queue_flag_set_unlocked(QUEUE_FLAG_NONROT, mq->queue);

 #ifdef CONFIG_MMC_BLOCK_BOUNCE
Index: work/drivers/s390/block/dasd.c
===================================================================
--- work.orig/drivers/s390/block/dasd.c
+++ work/drivers/s390/block/dasd.c
@@ -2196,7 +2196,7 @@ static void dasd_setup_queue(struct dasd
 	 */
 	blk_queue_max_segment_size(block->request_queue, PAGE_SIZE);
 	blk_queue_segment_boundary(block->request_queue, PAGE_SIZE - 1);
-	blk_queue_ordered(block->request_queue, QUEUE_ORDERED_DRAIN, NULL);
+	blk_queue_ordered(block->request_queue, QUEUE_ORDERED_BAR, NULL);
 }

 /*
Index: work/include/linux/elevator.h
===================================================================
--- work.orig/include/linux/elevator.h
+++ work/include/linux/elevator.h
@@ -162,9 +162,9 @@ extern struct request *elv_rb_find(struc
  * Insertion selection
  */
 #define ELEVATOR_INSERT_FRONT	1
-#define ELEVATOR_INSERT_BACK	2
-#define ELEVATOR_INSERT_SORT	3
-#define ELEVATOR_INSERT_REQUEUE	4
+#define ELEVATOR_INSERT_ORDERED	2
+#define ELEVATOR_INSERT_BACK	3
+#define ELEVATOR_INSERT_SORT	4

 /*
  * return values from elevator_may_queue_fn
Index: work/drivers/block/pktcdvd.c
===================================================================
--- work.orig/drivers/block/pktcdvd.c
+++ work/drivers/block/pktcdvd.c
@@ -752,7 +752,6 @@ static int pkt_generic_packet(struct pkt

 	rq->timeout = 60*HZ;
 	rq->cmd_type = REQ_TYPE_BLOCK_PC;
-	rq->cmd_flags |= REQ_HARDBARRIER;
 	if (cgc->quiet)
 		rq->cmd_flags |= REQ_QUIET;

Re: [PATCH, RFC] relaxed barriers

[Christoph Hellwig]

> Christoph, does this look like something the filesystems can use or
> have I misunderstood something?

This sounds very useful.  I'll review and test it once I get a bit time.

Re: [PATCH REPOST RFC] relaxed barriers

[Tejun Heo]

The patch was on top of v2.6.35 but was generated against dirty tree
and wouldn't apply cleanly.  Here's the proper one.

Thanks.
---
 block/blk-barrier.c          |  255 +++++++++++++++----------------------------
 block/blk-core.c             |   31 ++---
 block/blk.h                  |    5
 block/elevator.c             |   80 +------------
 drivers/block/brd.c          |    2
 drivers/block/loop.c         |    2
 drivers/block/osdblk.c       |    2
 drivers/block/pktcdvd.c      |    1
 drivers/block/ps3disk.c      |    3
 drivers/block/virtio_blk.c   |    4
 drivers/block/xen-blkfront.c |    2
 drivers/ide/ide-disk.c       |    4
 drivers/md/dm.c              |    3
 drivers/mmc/card/queue.c     |    2
 drivers/s390/block/dasd.c    |    2
 drivers/scsi/sd.c            |    8 -
 include/linux/blkdev.h       |   63 +++-------
 include/linux/elevator.h     |    6 -
 18 files changed, 155 insertions(+), 320 deletions(-)

Index: work/block/blk-barrier.c
===================================================================
--- work.orig/block/blk-barrier.c
+++ work/block/blk-barrier.c
@@ -9,6 +9,8 @@

 #include "blk.h"

+static struct request *queue_next_ordseq(struct request_queue *q);
+
 /**
  * blk_queue_ordered - does this queue support ordered writes
  * @q:        the request queue
@@ -31,13 +33,8 @@ int blk_queue_ordered(struct request_que
 		return -EINVAL;
 	}

-	if (ordered != QUEUE_ORDERED_NONE &&
-	    ordered != QUEUE_ORDERED_DRAIN &&
-	    ordered != QUEUE_ORDERED_DRAIN_FLUSH &&
-	    ordered != QUEUE_ORDERED_DRAIN_FUA &&
-	    ordered != QUEUE_ORDERED_TAG &&
-	    ordered != QUEUE_ORDERED_TAG_FLUSH &&
-	    ordered != QUEUE_ORDERED_TAG_FUA) {
+	if (ordered != QUEUE_ORDERED_NONE && ordered != QUEUE_ORDERED_BAR &&
+	    ordered != QUEUE_ORDERED_FLUSH && ordered != QUEUE_ORDERED_FUA) {
 		printk(KERN_ERR "blk_queue_ordered: bad value %d\n", ordered);
 		return -EINVAL;
 	}
@@ -60,38 +57,10 @@ unsigned blk_ordered_cur_seq(struct requ
 	return 1 << ffz(q->ordseq);
 }

-unsigned blk_ordered_req_seq(struct request *rq)
+static struct request *blk_ordered_complete_seq(struct request_queue *q,
+						unsigned seq, int error)
 {
-	struct request_queue *q = rq->q;
-
-	BUG_ON(q->ordseq == 0);
-
-	if (rq == &q->pre_flush_rq)
-		return QUEUE_ORDSEQ_PREFLUSH;
-	if (rq == &q->bar_rq)
-		return QUEUE_ORDSEQ_BAR;
-	if (rq == &q->post_flush_rq)
-		return QUEUE_ORDSEQ_POSTFLUSH;
-
-	/*
-	 * !fs requests don't need to follow barrier ordering.  Always
-	 * put them at the front.  This fixes the following deadlock.
-	 *
-	 * http://thread.gmane.org/gmane.linux.kernel/537473
-	 */
-	if (!blk_fs_request(rq))
-		return QUEUE_ORDSEQ_DRAIN;
-
-	if ((rq->cmd_flags & REQ_ORDERED_COLOR) ==
-	    (q->orig_bar_rq->cmd_flags & REQ_ORDERED_COLOR))
-		return QUEUE_ORDSEQ_DRAIN;
-	else
-		return QUEUE_ORDSEQ_DONE;
-}
-
-bool blk_ordered_complete_seq(struct request_queue *q, unsigned seq, int error)
-{
-	struct request *rq;
+	struct request *rq = NULL;

 	if (error && !q->orderr)
 		q->orderr = error;
@@ -99,16 +68,22 @@ bool blk_ordered_complete_seq(struct req
 	BUG_ON(q->ordseq & seq);
 	q->ordseq |= seq;

-	if (blk_ordered_cur_seq(q) != QUEUE_ORDSEQ_DONE)
-		return false;
-
-	/*
-	 * Okay, sequence complete.
-	 */
-	q->ordseq = 0;
-	rq = q->orig_bar_rq;
-	__blk_end_request_all(rq, q->orderr);
-	return true;
+	if (blk_ordered_cur_seq(q) != QUEUE_ORDSEQ_DONE) {
+		/* not complete yet, queue the next ordered sequence */
+		rq = queue_next_ordseq(q);
+	} else {
+		/* complete this barrier request */
+		__blk_end_request_all(q->orig_bar_rq, q->orderr);
+		q->orig_bar_rq = NULL;
+		q->ordseq = 0;
+
+		/* dispatch the next barrier if there's one */
+		if (!list_empty(&q->pending_barriers)) {
+			rq = list_entry_rq(q->pending_barriers.next);
+			list_move(&rq->queuelist, &q->queue_head);
+		}
+	}
+	return rq;
 }

 static void pre_flush_end_io(struct request *rq, int error)
@@ -129,21 +104,10 @@ static void post_flush_end_io(struct req
 	blk_ordered_complete_seq(rq->q, QUEUE_ORDSEQ_POSTFLUSH, error);
 }

-static void queue_flush(struct request_queue *q, unsigned which)
+static void queue_flush(struct request_queue *q, struct request *rq,
+			rq_end_io_fn *end_io)
 {
-	struct request *rq;
-	rq_end_io_fn *end_io;
-
-	if (which == QUEUE_ORDERED_DO_PREFLUSH) {
-		rq = &q->pre_flush_rq;
-		end_io = pre_flush_end_io;
-	} else {
-		rq = &q->post_flush_rq;
-		end_io = post_flush_end_io;
-	}
-
 	blk_rq_init(q, rq);
-	rq->cmd_flags = REQ_HARDBARRIER;
 	rq->rq_disk = q->bar_rq.rq_disk;
 	rq->end_io = end_io;
 	q->prepare_flush_fn(q, rq);
@@ -151,132 +115,93 @@ static void queue_flush(struct request_q
 	elv_insert(q, rq, ELEVATOR_INSERT_FRONT);
 }

-static inline bool start_ordered(struct request_queue *q, struct request **rqp)
+static struct request *queue_next_ordseq(struct request_queue *q)
 {
-	struct request *rq = *rqp;
-	unsigned skip = 0;
+	struct request *rq = &q->bar_rq;

-	q->orderr = 0;
-	q->ordered = q->next_ordered;
-	q->ordseq |= QUEUE_ORDSEQ_STARTED;
-
-	/*
-	 * For an empty barrier, there's no actual BAR request, which
-	 * in turn makes POSTFLUSH unnecessary.  Mask them off.
-	 */
-	if (!blk_rq_sectors(rq)) {
-		q->ordered &= ~(QUEUE_ORDERED_DO_BAR |
-				QUEUE_ORDERED_DO_POSTFLUSH);
-		/*
-		 * Empty barrier on a write-through device w/ ordered
-		 * tag has no command to issue and without any command
-		 * to issue, ordering by tag can't be used.  Drain
-		 * instead.
-		 */
-		if ((q->ordered & QUEUE_ORDERED_BY_TAG) &&
-		    !(q->ordered & QUEUE_ORDERED_DO_PREFLUSH)) {
-			q->ordered &= ~QUEUE_ORDERED_BY_TAG;
-			q->ordered |= QUEUE_ORDERED_BY_DRAIN;
-		}
-	}
-
-	/* stash away the original request */
-	blk_dequeue_request(rq);
-	q->orig_bar_rq = rq;
-	rq = NULL;
-
-	/*
-	 * Queue ordered sequence.  As we stack them at the head, we
-	 * need to queue in reverse order.  Note that we rely on that
-	 * no fs request uses ELEVATOR_INSERT_FRONT and thus no fs
-	 * request gets inbetween ordered sequence.
-	 */
-	if (q->ordered & QUEUE_ORDERED_DO_POSTFLUSH) {
-		queue_flush(q, QUEUE_ORDERED_DO_POSTFLUSH);
-		rq = &q->post_flush_rq;
-	} else
-		skip |= QUEUE_ORDSEQ_POSTFLUSH;
-
-	if (q->ordered & QUEUE_ORDERED_DO_BAR) {
-		rq = &q->bar_rq;
+	switch (blk_ordered_cur_seq(q)) {
+	case QUEUE_ORDSEQ_PREFLUSH:
+		queue_flush(q, rq, pre_flush_end_io);
+		break;

+	case QUEUE_ORDSEQ_BAR:
 		/* initialize proxy request and queue it */
 		blk_rq_init(q, rq);
-		if (bio_data_dir(q->orig_bar_rq->bio) == WRITE)
-			rq->cmd_flags |= REQ_RW;
+		init_request_from_bio(rq, q->orig_bar_rq->bio);
+		rq->cmd_flags &= ~REQ_HARDBARRIER;
 		if (q->ordered & QUEUE_ORDERED_DO_FUA)
 			rq->cmd_flags |= REQ_FUA;
-		init_request_from_bio(rq, q->orig_bar_rq->bio);
 		rq->end_io = bar_end_io;

 		elv_insert(q, rq, ELEVATOR_INSERT_FRONT);
-	} else
-		skip |= QUEUE_ORDSEQ_BAR;
+		break;

-	if (q->ordered & QUEUE_ORDERED_DO_PREFLUSH) {
-		queue_flush(q, QUEUE_ORDERED_DO_PREFLUSH);
-		rq = &q->pre_flush_rq;
-	} else
-		skip |= QUEUE_ORDSEQ_PREFLUSH;
+	case QUEUE_ORDSEQ_POSTFLUSH:
+		queue_flush(q, rq, post_flush_end_io);
+		break;

-	if ((q->ordered & QUEUE_ORDERED_BY_DRAIN) && queue_in_flight(q))
-		rq = NULL;
-	else
-		skip |= QUEUE_ORDSEQ_DRAIN;
-
-	*rqp = rq;
-
-	/*
-	 * Complete skipped sequences.  If whole sequence is complete,
-	 * return false to tell elevator that this request is gone.
-	 */
-	return !blk_ordered_complete_seq(q, skip, 0);
+	default:
+		BUG();
+	}
+	return rq;
 }

-bool blk_do_ordered(struct request_queue *q, struct request **rqp)
+struct request *blk_do_ordered(struct request_queue *q, struct request *rq)
 {
-	struct request *rq = *rqp;
-	const int is_barrier = blk_fs_request(rq) && blk_barrier_rq(rq);
+	unsigned skip = 0;

-	if (!q->ordseq) {
-		if (!is_barrier)
-			return true;
-
-		if (q->next_ordered != QUEUE_ORDERED_NONE)
-			return start_ordered(q, rqp);
-		else {
-			/*
-			 * Queue ordering not supported.  Terminate
-			 * with prejudice.
-			 */
-			blk_dequeue_request(rq);
-			__blk_end_request_all(rq, -EOPNOTSUPP);
-			*rqp = NULL;
-			return false;
-		}
+	if (!blk_barrier_rq(rq))
+		return rq;
+
+	if (q->ordseq) {
+		/*
+		 * Barrier is already in progress and they can't be
+		 * processed in parallel.  Queue for later processing.
+		 */
+		list_move_tail(&rq->queuelist, &q->pending_barriers);
+		return NULL;
+	}
+
+	if (unlikely(q->next_ordered == QUEUE_ORDERED_NONE)) {
+		/*
+		 * Queue ordering not supported.  Terminate
+		 * with prejudice.
+		 */
+		blk_dequeue_request(rq);
+		__blk_end_request_all(rq, -EOPNOTSUPP);
+		return NULL;
 	}

 	/*
-	 * Ordered sequence in progress
+	 * Start a new ordered sequence
 	 */
+	q->orderr = 0;
+	q->ordered = q->next_ordered;
+	q->ordseq |= QUEUE_ORDSEQ_STARTED;

-	/* Special requests are not subject to ordering rules. */
-	if (!blk_fs_request(rq) &&
-	    rq != &q->pre_flush_rq && rq != &q->post_flush_rq)
-		return true;
-
-	if (q->ordered & QUEUE_ORDERED_BY_TAG) {
-		/* Ordered by tag.  Blocking the next barrier is enough. */
-		if (is_barrier && rq != &q->bar_rq)
-			*rqp = NULL;
-	} else {
-		/* Ordered by draining.  Wait for turn. */
-		WARN_ON(blk_ordered_req_seq(rq) < blk_ordered_cur_seq(q));
-		if (blk_ordered_req_seq(rq) > blk_ordered_cur_seq(q))
-			*rqp = NULL;
-	}
+	/*
+	 * For an empty barrier, there's no actual BAR request, which
+	 * in turn makes POSTFLUSH unnecessary.  Mask them off.
+	 */
+	if (!blk_rq_sectors(rq))
+		q->ordered &= ~(QUEUE_ORDERED_DO_BAR |
+				QUEUE_ORDERED_DO_POSTFLUSH);
+
+	/* stash away the original request */
+	blk_dequeue_request(rq);
+	q->orig_bar_rq = rq;
+
+	if (!(q->ordered & QUEUE_ORDERED_DO_PREFLUSH))
+		skip |= QUEUE_ORDSEQ_PREFLUSH;
+
+	if (!(q->ordered & QUEUE_ORDERED_DO_BAR))
+		skip |= QUEUE_ORDSEQ_BAR;
+
+	if (!(q->ordered & QUEUE_ORDERED_DO_POSTFLUSH))
+		skip |= QUEUE_ORDSEQ_POSTFLUSH;

-	return true;
+	/* complete skipped sequences and return the first sequence */
+	return blk_ordered_complete_seq(q, skip, 0);
 }

 static void bio_end_empty_barrier(struct bio *bio, int err)
Index: work/include/linux/blkdev.h
===================================================================
--- work.orig/include/linux/blkdev.h
+++ work/include/linux/blkdev.h
@@ -106,7 +106,6 @@ enum rq_flag_bits {
 	__REQ_FAILED,		/* set if the request failed */
 	__REQ_QUIET,		/* don't worry about errors */
 	__REQ_PREEMPT,		/* set for "ide_preempt" requests */
-	__REQ_ORDERED_COLOR,	/* is before or after barrier */
 	__REQ_RW_SYNC,		/* request is sync (sync write or read) */
 	__REQ_ALLOCED,		/* request came from our alloc pool */
 	__REQ_RW_META,		/* metadata io request */
@@ -135,7 +134,6 @@ enum rq_flag_bits {
 #define REQ_FAILED	(1 << __REQ_FAILED)
 #define REQ_QUIET	(1 << __REQ_QUIET)
 #define REQ_PREEMPT	(1 << __REQ_PREEMPT)
-#define REQ_ORDERED_COLOR	(1 << __REQ_ORDERED_COLOR)
 #define REQ_RW_SYNC	(1 << __REQ_RW_SYNC)
 #define REQ_ALLOCED	(1 << __REQ_ALLOCED)
 #define REQ_RW_META	(1 << __REQ_RW_META)
@@ -437,9 +435,10 @@ struct request_queue
 	 * reserved for flush operations
 	 */
 	unsigned int		ordered, next_ordered, ordseq;
-	int			orderr, ordcolor;
-	struct request		pre_flush_rq, bar_rq, post_flush_rq;
-	struct request		*orig_bar_rq;
+	int			orderr;
+	struct request		bar_rq;
+	struct request          *orig_bar_rq;
+	struct list_head	pending_barriers;

 	struct mutex		sysfs_lock;

@@ -543,49 +542,33 @@ enum {
 	 * Hardbarrier is supported with one of the following methods.
 	 *
 	 * NONE		: hardbarrier unsupported
-	 * DRAIN	: ordering by draining is enough
-	 * DRAIN_FLUSH	: ordering by draining w/ pre and post flushes
-	 * DRAIN_FUA	: ordering by draining w/ pre flush and FUA write
-	 * TAG		: ordering by tag is enough
-	 * TAG_FLUSH	: ordering by tag w/ pre and post flushes
-	 * TAG_FUA	: ordering by tag w/ pre flush and FUA write
-	 */
-	QUEUE_ORDERED_BY_DRAIN		= 0x01,
-	QUEUE_ORDERED_BY_TAG		= 0x02,
-	QUEUE_ORDERED_DO_PREFLUSH	= 0x10,
-	QUEUE_ORDERED_DO_BAR		= 0x20,
-	QUEUE_ORDERED_DO_POSTFLUSH	= 0x40,
-	QUEUE_ORDERED_DO_FUA		= 0x80,
-
-	QUEUE_ORDERED_NONE		= 0x00,
-
-	QUEUE_ORDERED_DRAIN		= QUEUE_ORDERED_BY_DRAIN |
-					  QUEUE_ORDERED_DO_BAR,
-	QUEUE_ORDERED_DRAIN_FLUSH	= QUEUE_ORDERED_DRAIN |
-					  QUEUE_ORDERED_DO_PREFLUSH |
-					  QUEUE_ORDERED_DO_POSTFLUSH,
-	QUEUE_ORDERED_DRAIN_FUA		= QUEUE_ORDERED_DRAIN |
-					  QUEUE_ORDERED_DO_PREFLUSH |
-					  QUEUE_ORDERED_DO_FUA,
+	 * BAR		: writing out barrier is enough
+	 * FLUSH	: barrier and surrounding pre and post flushes
+	 * FUA		: FUA barrier w/ pre flush
+	 */
+	QUEUE_ORDERED_DO_PREFLUSH	= 1 << 0,
+	QUEUE_ORDERED_DO_BAR		= 1 << 1,
+	QUEUE_ORDERED_DO_POSTFLUSH	= 1 << 2,
+	QUEUE_ORDERED_DO_FUA		= 1 << 3,
+
+	QUEUE_ORDERED_NONE		= 0,

-	QUEUE_ORDERED_TAG		= QUEUE_ORDERED_BY_TAG |
-					  QUEUE_ORDERED_DO_BAR,
-	QUEUE_ORDERED_TAG_FLUSH		= QUEUE_ORDERED_TAG |
+	QUEUE_ORDERED_BAR		= QUEUE_ORDERED_DO_BAR,
+	QUEUE_ORDERED_FLUSH		= QUEUE_ORDERED_DO_BAR |
 					  QUEUE_ORDERED_DO_PREFLUSH |
 					  QUEUE_ORDERED_DO_POSTFLUSH,
-	QUEUE_ORDERED_TAG_FUA		= QUEUE_ORDERED_TAG |
+	QUEUE_ORDERED_FUA		= QUEUE_ORDERED_DO_BAR |
 					  QUEUE_ORDERED_DO_PREFLUSH |
 					  QUEUE_ORDERED_DO_FUA,

 	/*
 	 * Ordered operation sequence
 	 */
-	QUEUE_ORDSEQ_STARTED	= 0x01,	/* flushing in progress */
-	QUEUE_ORDSEQ_DRAIN	= 0x02,	/* waiting for the queue to be drained */
-	QUEUE_ORDSEQ_PREFLUSH	= 0x04,	/* pre-flushing in progress */
-	QUEUE_ORDSEQ_BAR	= 0x08,	/* original barrier req in progress */
-	QUEUE_ORDSEQ_POSTFLUSH	= 0x10,	/* post-flushing in progress */
-	QUEUE_ORDSEQ_DONE	= 0x20,
+	QUEUE_ORDSEQ_STARTED	= (1 << 0), /* flushing in progress */
+	QUEUE_ORDSEQ_PREFLUSH	= (1 << 1), /* pre-flushing in progress */
+	QUEUE_ORDSEQ_BAR	= (1 << 2), /* barrier write in progress */
+	QUEUE_ORDSEQ_POSTFLUSH	= (1 << 3), /* post-flushing in progress */
+	QUEUE_ORDSEQ_DONE	= (1 << 4),
 };

 #define blk_queue_plugged(q)	test_bit(QUEUE_FLAG_PLUGGED, &(q)->queue_flags)
@@ -967,10 +950,8 @@ extern void blk_queue_rq_timed_out(struc
 extern void blk_queue_rq_timeout(struct request_queue *, unsigned int);
 extern struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev);
 extern int blk_queue_ordered(struct request_queue *, unsigned, prepare_flush_fn *);
-extern bool blk_do_ordered(struct request_queue *, struct request **);
 extern unsigned blk_ordered_cur_seq(struct request_queue *);
 extern unsigned blk_ordered_req_seq(struct request *);
-extern bool blk_ordered_complete_seq(struct request_queue *, unsigned, int);

 extern int blk_rq_map_sg(struct request_queue *, struct request *, struct scatterlist *);
 extern void blk_dump_rq_flags(struct request *, char *);
Index: work/drivers/block/brd.c
===================================================================
--- work.orig/drivers/block/brd.c
+++ work/drivers/block/brd.c
@@ -479,7 +479,7 @@ static struct brd_device *brd_alloc(int
 	if (!brd->brd_queue)
 		goto out_free_dev;
 	blk_queue_make_request(brd->brd_queue, brd_make_request);
-	blk_queue_ordered(brd->brd_queue, QUEUE_ORDERED_TAG, NULL);
+	blk_queue_ordered(brd->brd_queue, QUEUE_ORDERED_BAR, NULL);
 	blk_queue_max_hw_sectors(brd->brd_queue, 1024);
 	blk_queue_bounce_limit(brd->brd_queue, BLK_BOUNCE_ANY);

Index: work/drivers/block/virtio_blk.c
===================================================================
--- work.orig/drivers/block/virtio_blk.c
+++ work/drivers/block/virtio_blk.c
@@ -368,10 +368,10 @@ static int __devinit virtblk_probe(struc

 	/* If barriers are supported, tell block layer that queue is ordered */
 	if (virtio_has_feature(vdev, VIRTIO_BLK_F_FLUSH))
-		blk_queue_ordered(q, QUEUE_ORDERED_DRAIN_FLUSH,
+		blk_queue_ordered(q, QUEUE_ORDERED_FLUSH,
 				  virtblk_prepare_flush);
 	else if (virtio_has_feature(vdev, VIRTIO_BLK_F_BARRIER))
-		blk_queue_ordered(q, QUEUE_ORDERED_TAG, NULL);
+		blk_queue_ordered(q, QUEUE_ORDERED_BAR, NULL);

 	/* If disk is read-only in the host, the guest should obey */
 	if (virtio_has_feature(vdev, VIRTIO_BLK_F_RO))
Index: work/drivers/scsi/sd.c
===================================================================
--- work.orig/drivers/scsi/sd.c
+++ work/drivers/scsi/sd.c
@@ -2103,15 +2103,13 @@ static int sd_revalidate_disk(struct gen

 	/*
 	 * We now have all cache related info, determine how we deal
-	 * with ordered requests.  Note that as the current SCSI
-	 * dispatch function can alter request order, we cannot use
-	 * QUEUE_ORDERED_TAG_* even when ordered tag is supported.
+	 * with ordered requests.
 	 */
 	if (sdkp->WCE)
 		ordered = sdkp->DPOFUA
-			? QUEUE_ORDERED_DRAIN_FUA : QUEUE_ORDERED_DRAIN_FLUSH;
+			? QUEUE_ORDERED_FUA : QUEUE_ORDERED_FLUSH;
 	else
-		ordered = QUEUE_ORDERED_DRAIN;
+		ordered = QUEUE_ORDERED_BAR;

 	blk_queue_ordered(sdkp->disk->queue, ordered, sd_prepare_flush);

Index: work/block/blk-core.c
===================================================================
--- work.orig/block/blk-core.c
+++ work/block/blk-core.c
@@ -520,6 +520,7 @@ struct request_queue *blk_alloc_queue_no
 	init_timer(&q->unplug_timer);
 	setup_timer(&q->timeout, blk_rq_timed_out_timer, (unsigned long) q);
 	INIT_LIST_HEAD(&q->timeout_list);
+	INIT_LIST_HEAD(&q->pending_barriers);
 	INIT_WORK(&q->unplug_work, blk_unplug_work);

 	kobject_init(&q->kobj, &blk_queue_ktype);
@@ -1036,22 +1037,6 @@ void blk_insert_request(struct request_q
 }
 EXPORT_SYMBOL(blk_insert_request);

-/*
- * add-request adds a request to the linked list.
- * queue lock is held and interrupts disabled, as we muck with the
- * request queue list.
- */
-static inline void add_request(struct request_queue *q, struct request *req)
-{
-	drive_stat_acct(req, 1);
-
-	/*
-	 * elevator indicated where it wants this request to be
-	 * inserted at elevator_merge time
-	 */
-	__elv_add_request(q, req, ELEVATOR_INSERT_SORT, 0);
-}
-
 static void part_round_stats_single(int cpu, struct hd_struct *part,
 				    unsigned long now)
 {
@@ -1184,6 +1169,7 @@ static int __make_request(struct request
 	const bool sync = bio_rw_flagged(bio, BIO_RW_SYNCIO);
 	const bool unplug = bio_rw_flagged(bio, BIO_RW_UNPLUG);
 	const unsigned int ff = bio->bi_rw & REQ_FAILFAST_MASK;
+	int where = ELEVATOR_INSERT_SORT;
 	int rw_flags;

 	if (bio_rw_flagged(bio, BIO_RW_BARRIER) &&
@@ -1191,6 +1177,7 @@ static int __make_request(struct request
 		bio_endio(bio, -EOPNOTSUPP);
 		return 0;
 	}
+
 	/*
 	 * low level driver can indicate that it wants pages above a
 	 * certain limit bounced to low memory (ie for highmem, or even
@@ -1200,7 +1187,12 @@ static int __make_request(struct request

 	spin_lock_irq(q->queue_lock);

-	if (unlikely(bio_rw_flagged(bio, BIO_RW_BARRIER)) || elv_queue_empty(q))
+	if (bio_rw_flagged(bio, BIO_RW_BARRIER)) {
+		where = ELEVATOR_INSERT_ORDERED;
+		goto get_rq;
+	}
+
+	if (elv_queue_empty(q))
 		goto get_rq;

 	el_ret = elv_merge(q, &req, bio);
@@ -1297,7 +1289,10 @@ get_rq:
 		req->cpu = blk_cpu_to_group(smp_processor_id());
 	if (queue_should_plug(q) && elv_queue_empty(q))
 		blk_plug_device(q);
-	add_request(q, req);
+
+	/* insert the request into the elevator */
+	drive_stat_acct(req, 1);
+	__elv_add_request(q, req, where, 0);
 out:
 	if (unplug || !queue_should_plug(q))
 		__generic_unplug_device(q);
Index: work/block/elevator.c
===================================================================
--- work.orig/block/elevator.c
+++ work/block/elevator.c
@@ -564,7 +564,7 @@ void elv_requeue_request(struct request_

 	rq->cmd_flags &= ~REQ_STARTED;

-	elv_insert(q, rq, ELEVATOR_INSERT_REQUEUE);
+	elv_insert(q, rq, ELEVATOR_INSERT_FRONT);
 }

 void elv_drain_elevator(struct request_queue *q)
@@ -611,8 +611,6 @@ void elv_quiesce_end(struct request_queu

 void elv_insert(struct request_queue *q, struct request *rq, int where)
 {
-	struct list_head *pos;
-	unsigned ordseq;
 	int unplug_it = 1;

 	trace_block_rq_insert(q, rq);
@@ -622,10 +620,14 @@ void elv_insert(struct request_queue *q,
 	switch (where) {
 	case ELEVATOR_INSERT_FRONT:
 		rq->cmd_flags |= REQ_SOFTBARRIER;
-
 		list_add(&rq->queuelist, &q->queue_head);
 		break;

+	case ELEVATOR_INSERT_ORDERED:
+		rq->cmd_flags |= REQ_SOFTBARRIER;
+		list_add_tail(&rq->queuelist, &q->queue_head);
+		break;
+
 	case ELEVATOR_INSERT_BACK:
 		rq->cmd_flags |= REQ_SOFTBARRIER;
 		elv_drain_elevator(q);
@@ -661,36 +663,6 @@ void elv_insert(struct request_queue *q,
 		q->elevator->ops->elevator_add_req_fn(q, rq);
 		break;

-	case ELEVATOR_INSERT_REQUEUE:
-		/*
-		 * If ordered flush isn't in progress, we do front
-		 * insertion; otherwise, requests should be requeued
-		 * in ordseq order.
-		 */
-		rq->cmd_flags |= REQ_SOFTBARRIER;
-
-		/*
-		 * Most requeues happen because of a busy condition,
-		 * don't force unplug of the queue for that case.
-		 */
-		unplug_it = 0;
-
-		if (q->ordseq == 0) {
-			list_add(&rq->queuelist, &q->queue_head);
-			break;
-		}
-
-		ordseq = blk_ordered_req_seq(rq);
-
-		list_for_each(pos, &q->queue_head) {
-			struct request *pos_rq = list_entry_rq(pos);
-			if (ordseq <= blk_ordered_req_seq(pos_rq))
-				break;
-		}
-
-		list_add_tail(&rq->queuelist, pos);
-		break;
-
 	default:
 		printk(KERN_ERR "%s: bad insertion point %d\n",
 		       __func__, where);
@@ -709,32 +681,14 @@ void elv_insert(struct request_queue *q,
 void __elv_add_request(struct request_queue *q, struct request *rq, int where,
 		       int plug)
 {
-	if (q->ordcolor)
-		rq->cmd_flags |= REQ_ORDERED_COLOR;
-
 	if (rq->cmd_flags & (REQ_SOFTBARRIER | REQ_HARDBARRIER)) {
-		/*
-		 * toggle ordered color
-		 */
-		if (blk_barrier_rq(rq))
-			q->ordcolor ^= 1;
-
-		/*
-		 * barriers implicitly indicate back insertion
-		 */
-		if (where == ELEVATOR_INSERT_SORT)
-			where = ELEVATOR_INSERT_BACK;
-
-		/*
-		 * this request is scheduling boundary, update
-		 * end_sector
-		 */
+		/* barriers are scheduling boundary, update end_sector */
 		if (blk_fs_request(rq) || blk_discard_rq(rq)) {
 			q->end_sector = rq_end_sector(rq);
 			q->boundary_rq = rq;
 		}
 	} else if (!(rq->cmd_flags & REQ_ELVPRIV) &&
-		    where == ELEVATOR_INSERT_SORT)
+		   where == ELEVATOR_INSERT_SORT)
 		where = ELEVATOR_INSERT_BACK;

 	if (plug)
@@ -846,24 +800,6 @@ void elv_completed_request(struct reques
 		if (blk_sorted_rq(rq) && e->ops->elevator_completed_req_fn)
 			e->ops->elevator_completed_req_fn(q, rq);
 	}
-
-	/*
-	 * Check if the queue is waiting for fs requests to be
-	 * drained for flush sequence.
-	 */
-	if (unlikely(q->ordseq)) {
-		struct request *next = NULL;
-
-		if (!list_empty(&q->queue_head))
-			next = list_entry_rq(q->queue_head.next);
-
-		if (!queue_in_flight(q) &&
-		    blk_ordered_cur_seq(q) == QUEUE_ORDSEQ_DRAIN &&
-		    (!next || blk_ordered_req_seq(next) > QUEUE_ORDSEQ_DRAIN)) {
-			blk_ordered_complete_seq(q, QUEUE_ORDSEQ_DRAIN, 0);
-			__blk_run_queue(q);
-		}
-	}
 }

 #define to_elv(atr) container_of((atr), struct elv_fs_entry, attr)
Index: work/block/blk.h
===================================================================
--- work.orig/block/blk.h
+++ work/block/blk.h
@@ -51,6 +51,8 @@ static inline void blk_clear_rq_complete
  */
 #define ELV_ON_HASH(rq)		(!hlist_unhashed(&(rq)->hash))

+struct request *blk_do_ordered(struct request_queue *q, struct request *rq);
+
 static inline struct request *__elv_next_request(struct request_queue *q)
 {
 	struct request *rq;
@@ -58,7 +60,8 @@ static inline struct request *__elv_next
 	while (1) {
 		while (!list_empty(&q->queue_head)) {
 			rq = list_entry_rq(q->queue_head.next);
-			if (blk_do_ordered(q, &rq))
+			rq = blk_do_ordered(q, rq);
+			if (rq)
 				return rq;
 		}

Index: work/drivers/block/loop.c
===================================================================
--- work.orig/drivers/block/loop.c
+++ work/drivers/block/loop.c
@@ -831,7 +831,7 @@ static int loop_set_fd(struct loop_devic
 	lo->lo_queue->unplug_fn = loop_unplug;

 	if (!(lo_flags & LO_FLAGS_READ_ONLY) && file->f_op->fsync)
-		blk_queue_ordered(lo->lo_queue, QUEUE_ORDERED_DRAIN, NULL);
+		blk_queue_ordered(lo->lo_queue, QUEUE_ORDERED_BAR, NULL);

 	set_capacity(lo->lo_disk, size);
 	bd_set_size(bdev, size << 9);
Index: work/drivers/block/osdblk.c
===================================================================
--- work.orig/drivers/block/osdblk.c
+++ work/drivers/block/osdblk.c
@@ -446,7 +446,7 @@ static int osdblk_init_disk(struct osdbl
 	blk_queue_stack_limits(q, osd_request_queue(osdev->osd));

 	blk_queue_prep_rq(q, blk_queue_start_tag);
-	blk_queue_ordered(q, QUEUE_ORDERED_DRAIN_FLUSH, osdblk_prepare_flush);
+	blk_queue_ordered(q, QUEUE_ORDERED_FLUSH, osdblk_prepare_flush);

 	disk->queue = q;

Index: work/drivers/block/ps3disk.c
===================================================================
--- work.orig/drivers/block/ps3disk.c
+++ work/drivers/block/ps3disk.c
@@ -480,8 +480,7 @@ static int __devinit ps3disk_probe(struc
 	blk_queue_dma_alignment(queue, dev->blk_size-1);
 	blk_queue_logical_block_size(queue, dev->blk_size);

-	blk_queue_ordered(queue, QUEUE_ORDERED_DRAIN_FLUSH,
-			  ps3disk_prepare_flush);
+	blk_queue_ordered(queue, QUEUE_ORDERED_FLUSH, ps3disk_prepare_flush);

 	blk_queue_max_segments(queue, -1);
 	blk_queue_max_segment_size(queue, dev->bounce_size);
Index: work/drivers/block/xen-blkfront.c
===================================================================
--- work.orig/drivers/block/xen-blkfront.c
+++ work/drivers/block/xen-blkfront.c
@@ -373,7 +373,7 @@ static int xlvbd_barrier(struct blkfront
 	int err;

 	err = blk_queue_ordered(info->rq,
-				info->feature_barrier ? QUEUE_ORDERED_DRAIN : QUEUE_ORDERED_NONE,
+				info->feature_barrier ? QUEUE_ORDERED_BAR : QUEUE_ORDERED_NONE,
 				NULL);

 	if (err)
Index: work/drivers/ide/ide-disk.c
===================================================================
--- work.orig/drivers/ide/ide-disk.c
+++ work/drivers/ide/ide-disk.c
@@ -537,11 +537,11 @@ static void update_ordered(ide_drive_t *
 		       drive->name, barrier ? "" : "not ");

 		if (barrier) {
-			ordered = QUEUE_ORDERED_DRAIN_FLUSH;
+			ordered = QUEUE_ORDERED_FLUSH;
 			prep_fn = idedisk_prepare_flush;
 		}
 	} else
-		ordered = QUEUE_ORDERED_DRAIN;
+		ordered = QUEUE_ORDERED_BAR;

 	blk_queue_ordered(drive->queue, ordered, prep_fn);
 }
Index: work/drivers/md/dm.c
===================================================================
--- work.orig/drivers/md/dm.c
+++ work/drivers/md/dm.c
@@ -1912,8 +1912,7 @@ static struct mapped_device *alloc_dev(i
 	blk_queue_softirq_done(md->queue, dm_softirq_done);
 	blk_queue_prep_rq(md->queue, dm_prep_fn);
 	blk_queue_lld_busy(md->queue, dm_lld_busy);
-	blk_queue_ordered(md->queue, QUEUE_ORDERED_DRAIN_FLUSH,
-			  dm_rq_prepare_flush);
+	blk_queue_ordered(md->queue, QUEUE_ORDERED_FLUSH, dm_rq_prepare_flush);

 	md->disk = alloc_disk(1);
 	if (!md->disk)
Index: work/drivers/mmc/card/queue.c
===================================================================
--- work.orig/drivers/mmc/card/queue.c
+++ work/drivers/mmc/card/queue.c
@@ -128,7 +128,7 @@ int mmc_init_queue(struct mmc_queue *mq,
 	mq->req = NULL;

 	blk_queue_prep_rq(mq->queue, mmc_prep_request);
-	blk_queue_ordered(mq->queue, QUEUE_ORDERED_DRAIN, NULL);
+	blk_queue_ordered(mq->queue, QUEUE_ORDERED_BAR, NULL);
 	queue_flag_set_unlocked(QUEUE_FLAG_NONROT, mq->queue);

 #ifdef CONFIG_MMC_BLOCK_BOUNCE
Index: work/drivers/s390/block/dasd.c
===================================================================
--- work.orig/drivers/s390/block/dasd.c
+++ work/drivers/s390/block/dasd.c
@@ -2196,7 +2196,7 @@ static void dasd_setup_queue(struct dasd
 	 */
 	blk_queue_max_segment_size(block->request_queue, PAGE_SIZE);
 	blk_queue_segment_boundary(block->request_queue, PAGE_SIZE - 1);
-	blk_queue_ordered(block->request_queue, QUEUE_ORDERED_DRAIN, NULL);
+	blk_queue_ordered(block->request_queue, QUEUE_ORDERED_BAR, NULL);
 }

 /*
Index: work/include/linux/elevator.h
===================================================================
--- work.orig/include/linux/elevator.h
+++ work/include/linux/elevator.h
@@ -162,9 +162,9 @@ extern struct request *elv_rb_find(struc
  * Insertion selection
  */
 #define ELEVATOR_INSERT_FRONT	1
-#define ELEVATOR_INSERT_BACK	2
-#define ELEVATOR_INSERT_SORT	3
-#define ELEVATOR_INSERT_REQUEUE	4
+#define ELEVATOR_INSERT_ORDERED	2
+#define ELEVATOR_INSERT_BACK	3
+#define ELEVATOR_INSERT_SORT	4

 /*
  * return values from elevator_may_queue_fn
Index: work/drivers/block/pktcdvd.c
===================================================================
--- work.orig/drivers/block/pktcdvd.c
+++ work/drivers/block/pktcdvd.c
@@ -752,7 +752,6 @@ static int pkt_generic_packet(struct pkt

 	rq->timeout = 60*HZ;
 	rq->cmd_type = REQ_TYPE_BLOCK_PC;
-	rq->cmd_flags |= REQ_HARDBARRIER;
 	if (cgc->quiet)
 		rq->cmd_flags |= REQ_QUIET;

Re: [PATCH REPOST RFC] relaxed barriers

[Christoph Hellwig]

On Sat, Aug 07, 2010 at 12:13:06PM +0200, Tejun Heo wrote:
> The patch was on top of v2.6.35 but was generated against dirty tree
> and wouldn't apply cleanly.  Here's the proper one.

Here's an updated version:

 (a) ported to Jens' current block tree
 (b) optimize barriers on devices not requiring flushes to be no-ops
 (b) redo the blk_queue_ordered interface to just set QUEUE_HAS_FLUSH
     and QUEUE_HAS_FUA flags.

Index: linux-2.6/block/blk-barrier.c
===================================================================
--- linux-2.6.orig/block/blk-barrier.c	2010-08-07 12:53:23.727479189 -0400
+++ linux-2.6/block/blk-barrier.c	2010-08-07 14:52:21.402479191 -0400
@@ -9,37 +9,36 @@
 
 #include "blk.h"
 
+/*
+ * Ordered operation sequence.
+ */
+enum {
+	QUEUE_ORDSEQ_STARTED	= (1 << 0), /* flushing in progress */
+	QUEUE_ORDSEQ_PREFLUSH	= (1 << 1), /* pre-flushing in progress */
+	QUEUE_ORDSEQ_BAR	= (1 << 2), /* barrier write in progress */
+	QUEUE_ORDSEQ_POSTFLUSH	= (1 << 3), /* post-flushing in progress */
+	QUEUE_ORDSEQ_DONE	= (1 << 4),
+};
+
+static struct request *queue_next_ordseq(struct request_queue *q);
+
 /**
- * blk_queue_ordered - does this queue support ordered writes
- * @q:        the request queue
- * @ordered:  one of QUEUE_ORDERED_*
- *
- * Description:
- *   For journalled file systems, doing ordered writes on a commit
- *   block instead of explicitly doing wait_on_buffer (which is bad
- *   for performance) can be a big win. Block drivers supporting this
- *   feature should call this function and indicate so.
- *
+ * blk_queue_cache_features - set the supported cache control features
+ * @q:        		the request queue
+ * @cache_features:	the support features
  **/
-int blk_queue_ordered(struct request_queue *q, unsigned ordered)
+int blk_queue_cache_features(struct request_queue *q, unsigned cache_features)
 {
-	if (ordered != QUEUE_ORDERED_NONE &&
-	    ordered != QUEUE_ORDERED_DRAIN &&
-	    ordered != QUEUE_ORDERED_DRAIN_FLUSH &&
-	    ordered != QUEUE_ORDERED_DRAIN_FUA &&
-	    ordered != QUEUE_ORDERED_TAG &&
-	    ordered != QUEUE_ORDERED_TAG_FLUSH &&
-	    ordered != QUEUE_ORDERED_TAG_FUA) {
-		printk(KERN_ERR "blk_queue_ordered: bad value %d\n", ordered);
+	if (cache_features & ~(QUEUE_HAS_FLUSH|QUEUE_HAS_FUA)) {
+		printk(KERN_ERR "blk_queue_cache_features: bad value %d\n",
+			cache_features);
 		return -EINVAL;
 	}
 
-	q->ordered = ordered;
-	q->next_ordered = ordered;
-
+	q->cache_features = cache_features;
 	return 0;
 }
-EXPORT_SYMBOL(blk_queue_ordered);
+EXPORT_SYMBOL(blk_queue_cache_features);
 
 /*
  * Cache flushing for ordered writes handling
@@ -51,38 +50,10 @@ unsigned blk_ordered_cur_seq(struct requ
 	return 1 << ffz(q->ordseq);
 }
 
-unsigned blk_ordered_req_seq(struct request *rq)
-{
-	struct request_queue *q = rq->q;
-
-	BUG_ON(q->ordseq == 0);
-
-	if (rq == &q->pre_flush_rq)
-		return QUEUE_ORDSEQ_PREFLUSH;
-	if (rq == &q->bar_rq)
-		return QUEUE_ORDSEQ_BAR;
-	if (rq == &q->post_flush_rq)
-		return QUEUE_ORDSEQ_POSTFLUSH;
-
-	/*
-	 * !fs requests don't need to follow barrier ordering.  Always
-	 * put them at the front.  This fixes the following deadlock.
-	 *
-	 * http://thread.gmane.org/gmane.linux.kernel/537473
-	 */
-	if (rq->cmd_type != REQ_TYPE_FS)
-		return QUEUE_ORDSEQ_DRAIN;
-
-	if ((rq->cmd_flags & REQ_ORDERED_COLOR) ==
-	    (q->orig_bar_rq->cmd_flags & REQ_ORDERED_COLOR))
-		return QUEUE_ORDSEQ_DRAIN;
-	else
-		return QUEUE_ORDSEQ_DONE;
-}
-
-bool blk_ordered_complete_seq(struct request_queue *q, unsigned seq, int error)
+static struct request *blk_ordered_complete_seq(struct request_queue *q,
+						unsigned seq, int error)
 {
-	struct request *rq;
+	struct request *rq = NULL;
 
 	if (error && !q->orderr)
 		q->orderr = error;
@@ -90,16 +61,22 @@ bool blk_ordered_complete_seq(struct req
 	BUG_ON(q->ordseq & seq);
 	q->ordseq |= seq;
 
-	if (blk_ordered_cur_seq(q) != QUEUE_ORDSEQ_DONE)
-		return false;
-
-	/*
-	 * Okay, sequence complete.
-	 */
-	q->ordseq = 0;
-	rq = q->orig_bar_rq;
-	__blk_end_request_all(rq, q->orderr);
-	return true;
+	if (blk_ordered_cur_seq(q) != QUEUE_ORDSEQ_DONE) {
+		/* not complete yet, queue the next ordered sequence */
+		rq = queue_next_ordseq(q);
+	} else {
+		/* complete this barrier request */
+		__blk_end_request_all(q->orig_bar_rq, q->orderr);
+		q->orig_bar_rq = NULL;
+		q->ordseq = 0;
+
+		/* dispatch the next barrier if there's one */
+		if (!list_empty(&q->pending_barriers)) {
+			rq = list_entry_rq(q->pending_barriers.next);
+			list_move(&rq->queuelist, &q->queue_head);
+		}
+	}
+	return rq;
 }
 
 static void pre_flush_end_io(struct request *rq, int error)
@@ -120,155 +97,100 @@ static void post_flush_end_io(struct req
 	blk_ordered_complete_seq(rq->q, QUEUE_ORDSEQ_POSTFLUSH, error);
 }
 
-static void queue_flush(struct request_queue *q, unsigned which)
+static void init_flush_request(struct request_queue *q, struct request *rq)
 {
-	struct request *rq;
-	rq_end_io_fn *end_io;
+	rq->cmd_type = REQ_TYPE_FS;
+	rq->cmd_flags = REQ_FLUSH;
+	rq->rq_disk = q->orig_bar_rq->rq_disk;
+}
 
-	if (which == QUEUE_ORDERED_DO_PREFLUSH) {
-		rq = &q->pre_flush_rq;
-		end_io = pre_flush_end_io;
-	} else {
-		rq = &q->post_flush_rq;
-		end_io = post_flush_end_io;
-	}
+/*
+ * Initialize proxy request and queue it.
+ */
+static struct request *queue_next_ordseq(struct request_queue *q)
+{
+	struct request *rq = &q->bar_rq;
 
 	blk_rq_init(q, rq);
-	rq->cmd_type = REQ_TYPE_FS;
-	rq->cmd_flags = REQ_HARDBARRIER | REQ_FLUSH;
-	rq->rq_disk = q->orig_bar_rq->rq_disk;
-	rq->end_io = end_io;
+
+	switch (blk_ordered_cur_seq(q)) {
+	case QUEUE_ORDSEQ_PREFLUSH:
+		init_flush_request(q, rq);
+		rq->end_io = pre_flush_end_io;
+		break;
+	case QUEUE_ORDSEQ_BAR:
+		init_request_from_bio(rq, q->orig_bar_rq->bio);
+		rq->cmd_flags &= ~REQ_HARDBARRIER;
+ 		if (q->cache_features & QUEUE_HAS_FUA)
+ 			rq->cmd_flags |= REQ_FUA;
+		rq->end_io = bar_end_io;
+		break;
+	case QUEUE_ORDSEQ_POSTFLUSH:
+		init_flush_request(q, rq);
+		rq->end_io = post_flush_end_io;
+		break;
+	default:
+		BUG();
+	}
 
 	elv_insert(q, rq, ELEVATOR_INSERT_FRONT);
+	return rq;
 }
 
-static inline bool start_ordered(struct request_queue *q, struct request **rqp)
+struct request *blk_do_ordered(struct request_queue *q, struct request *rq)
 {
-	struct request *rq = *rqp;
 	unsigned skip = 0;
 
-	q->orderr = 0;
-	q->ordered = q->next_ordered;
-	q->ordseq |= QUEUE_ORDSEQ_STARTED;
+	if (rq->cmd_type != REQ_TYPE_FS)
+		return rq;
+	if (!(rq->cmd_flags & REQ_HARDBARRIER))
+		return rq;
 
-	/*
-	 * For an empty barrier, there's no actual BAR request, which
-	 * in turn makes POSTFLUSH unnecessary.  Mask them off.
-	 */
-	if (!blk_rq_sectors(rq)) {
-		q->ordered &= ~(QUEUE_ORDERED_DO_BAR |
-				QUEUE_ORDERED_DO_POSTFLUSH);
+	if (!(q->cache_features & QUEUE_HAS_FLUSH)) {
 		/*
-		 * Empty barrier on a write-through device w/ ordered
-		 * tag has no command to issue and without any command
-		 * to issue, ordering by tag can't be used.  Drain
-		 * instead.
+		 * No flush required.  We can just send on write requests
+		 * and complete cache flush requests ASAP.
 		 */
-		if ((q->ordered & QUEUE_ORDERED_BY_TAG) &&
-		    !(q->ordered & QUEUE_ORDERED_DO_PREFLUSH)) {
-			q->ordered &= ~QUEUE_ORDERED_BY_TAG;
-			q->ordered |= QUEUE_ORDERED_BY_DRAIN;
+		if (blk_rq_sectors(rq)) {
+			rq->cmd_flags &= ~REQ_HARDBARRIER;
+			return rq;
 		}
+		blk_dequeue_request(rq);
+		__blk_end_request_all(rq, 0);
+		return NULL;
 	}
 
-	/* stash away the original request */
-	blk_dequeue_request(rq);
-	q->orig_bar_rq = rq;
-	rq = NULL;
-
-	/*
-	 * Queue ordered sequence.  As we stack them at the head, we
-	 * need to queue in reverse order.  Note that we rely on that
-	 * no fs request uses ELEVATOR_INSERT_FRONT and thus no fs
-	 * request gets inbetween ordered sequence.
-	 */
-	if (q->ordered & QUEUE_ORDERED_DO_POSTFLUSH) {
-		queue_flush(q, QUEUE_ORDERED_DO_POSTFLUSH);
-		rq = &q->post_flush_rq;
-	} else
-		skip |= QUEUE_ORDSEQ_POSTFLUSH;
-
-	if (q->ordered & QUEUE_ORDERED_DO_BAR) {
-		rq = &q->bar_rq;
-
-		/* initialize proxy request and queue it */
-		blk_rq_init(q, rq);
-		if (bio_data_dir(q->orig_bar_rq->bio) == WRITE)
-			rq->cmd_flags |= REQ_WRITE;
-		if (q->ordered & QUEUE_ORDERED_DO_FUA)
-			rq->cmd_flags |= REQ_FUA;
-		init_request_from_bio(rq, q->orig_bar_rq->bio);
-		rq->end_io = bar_end_io;
-
-		elv_insert(q, rq, ELEVATOR_INSERT_FRONT);
-	} else
-		skip |= QUEUE_ORDSEQ_BAR;
-
-	if (q->ordered & QUEUE_ORDERED_DO_PREFLUSH) {
-		queue_flush(q, QUEUE_ORDERED_DO_PREFLUSH);
-		rq = &q->pre_flush_rq;
-	} else
-		skip |= QUEUE_ORDSEQ_PREFLUSH;
-
-	if ((q->ordered & QUEUE_ORDERED_BY_DRAIN) && queue_in_flight(q))
-		rq = NULL;
-	else
-		skip |= QUEUE_ORDSEQ_DRAIN;
+	if (q->ordseq) {
+		/*
+		 * Barrier is already in progress and they can't be
+		 * processed in parallel.  Queue for later processing.
+		 */
+		list_move_tail(&rq->queuelist, &q->pending_barriers);
+		return NULL;
+	}
 
-	*rqp = rq;
 
 	/*
-	 * Complete skipped sequences.  If whole sequence is complete,
-	 * return false to tell elevator that this request is gone.
+	 * Start a new ordered sequence
 	 */
-	return !blk_ordered_complete_seq(q, skip, 0);
-}
-
-bool blk_do_ordered(struct request_queue *q, struct request **rqp)
-{
-	struct request *rq = *rqp;
-	const int is_barrier = rq->cmd_type == REQ_TYPE_FS &&
-				(rq->cmd_flags & REQ_HARDBARRIER);
-
-	if (!q->ordseq) {
-		if (!is_barrier)
-			return true;
-
-		if (q->next_ordered != QUEUE_ORDERED_NONE)
-			return start_ordered(q, rqp);
-		else {
-			/*
-			 * Queue ordering not supported.  Terminate
-			 * with prejudice.
-			 */
-			blk_dequeue_request(rq);
-			__blk_end_request_all(rq, -EOPNOTSUPP);
-			*rqp = NULL;
-			return false;
-		}
-	}
+	q->orderr = 0;
+	q->ordseq |= QUEUE_ORDSEQ_STARTED;
 
 	/*
-	 * Ordered sequence in progress
+	 * For an empty barrier, there's no actual BAR request, which
+	 * in turn makes POSTFLUSH unnecessary.  Mask them off.
 	 */
+	if (!blk_rq_sectors(rq))
+		skip |= (QUEUE_ORDSEQ_BAR|QUEUE_ORDSEQ_POSTFLUSH);
+	else if (q->cache_features & QUEUE_HAS_FUA)
+		skip |= QUEUE_ORDSEQ_POSTFLUSH;
 
-	/* Special requests are not subject to ordering rules. */
-	if (rq->cmd_type != REQ_TYPE_FS &&
-	    rq != &q->pre_flush_rq && rq != &q->post_flush_rq)
-		return true;
-
-	if (q->ordered & QUEUE_ORDERED_BY_TAG) {
-		/* Ordered by tag.  Blocking the next barrier is enough. */
-		if (is_barrier && rq != &q->bar_rq)
-			*rqp = NULL;
-	} else {
-		/* Ordered by draining.  Wait for turn. */
-		WARN_ON(blk_ordered_req_seq(rq) < blk_ordered_cur_seq(q));
-		if (blk_ordered_req_seq(rq) > blk_ordered_cur_seq(q))
-			*rqp = NULL;
-	}
+	/* stash away the original request */
+	blk_dequeue_request(rq);
+	q->orig_bar_rq = rq;
 
-	return true;
+	/* complete skipped sequences and return the first sequence */
+	return blk_ordered_complete_seq(q, skip, 0);
 }
 
 static void bio_end_empty_barrier(struct bio *bio, int err)
Index: linux-2.6/include/linux/blkdev.h
===================================================================
--- linux-2.6.orig/include/linux/blkdev.h	2010-08-07 12:53:23.774479189 -0400
+++ linux-2.6/include/linux/blkdev.h	2010-08-07 14:51:42.751479190 -0400
@@ -354,13 +354,20 @@ struct request_queue
 #ifdef CONFIG_BLK_DEV_IO_TRACE
 	struct blk_trace	*blk_trace;
 #endif
+
+	/*
+	 * Features this queue understands.
+	 */
+	unsigned int		cache_features;
+
 	/*
 	 * reserved for flush operations
 	 */
-	unsigned int		ordered, next_ordered, ordseq;
-	int			orderr, ordcolor;
-	struct request		pre_flush_rq, bar_rq, post_flush_rq;
-	struct request		*orig_bar_rq;
+	unsigned int		ordseq;
+	int			orderr;
+	struct request		bar_rq;
+	struct request          *orig_bar_rq;
+	struct list_head	pending_barriers;
 
 	struct mutex		sysfs_lock;
 
@@ -461,54 +468,12 @@ static inline void queue_flag_clear(unsi
 	__clear_bit(flag, &q->queue_flags);
 }
 
+/*
+ * Possible features to control a volatile write cache.
+ */
 enum {
-	/*
-	 * Hardbarrier is supported with one of the following methods.
-	 *
-	 * NONE		: hardbarrier unsupported
-	 * DRAIN	: ordering by draining is enough
-	 * DRAIN_FLUSH	: ordering by draining w/ pre and post flushes
-	 * DRAIN_FUA	: ordering by draining w/ pre flush and FUA write
-	 * TAG		: ordering by tag is enough
-	 * TAG_FLUSH	: ordering by tag w/ pre and post flushes
-	 * TAG_FUA	: ordering by tag w/ pre flush and FUA write
-	 */
-	QUEUE_ORDERED_BY_DRAIN		= 0x01,
-	QUEUE_ORDERED_BY_TAG		= 0x02,
-	QUEUE_ORDERED_DO_PREFLUSH	= 0x10,
-	QUEUE_ORDERED_DO_BAR		= 0x20,
-	QUEUE_ORDERED_DO_POSTFLUSH	= 0x40,
-	QUEUE_ORDERED_DO_FUA		= 0x80,
-
-	QUEUE_ORDERED_NONE		= 0x00,
-
-	QUEUE_ORDERED_DRAIN		= QUEUE_ORDERED_BY_DRAIN |
-					  QUEUE_ORDERED_DO_BAR,
-	QUEUE_ORDERED_DRAIN_FLUSH	= QUEUE_ORDERED_DRAIN |
-					  QUEUE_ORDERED_DO_PREFLUSH |
-					  QUEUE_ORDERED_DO_POSTFLUSH,
-	QUEUE_ORDERED_DRAIN_FUA		= QUEUE_ORDERED_DRAIN |
-					  QUEUE_ORDERED_DO_PREFLUSH |
-					  QUEUE_ORDERED_DO_FUA,
-
-	QUEUE_ORDERED_TAG		= QUEUE_ORDERED_BY_TAG |
-					  QUEUE_ORDERED_DO_BAR,
-	QUEUE_ORDERED_TAG_FLUSH		= QUEUE_ORDERED_TAG |
-					  QUEUE_ORDERED_DO_PREFLUSH |
-					  QUEUE_ORDERED_DO_POSTFLUSH,
-	QUEUE_ORDERED_TAG_FUA		= QUEUE_ORDERED_TAG |
-					  QUEUE_ORDERED_DO_PREFLUSH |
-					  QUEUE_ORDERED_DO_FUA,
-
-	/*
-	 * Ordered operation sequence
-	 */
-	QUEUE_ORDSEQ_STARTED	= 0x01,	/* flushing in progress */
-	QUEUE_ORDSEQ_DRAIN	= 0x02,	/* waiting for the queue to be drained */
-	QUEUE_ORDSEQ_PREFLUSH	= 0x04,	/* pre-flushing in progress */
-	QUEUE_ORDSEQ_BAR	= 0x08,	/* original barrier req in progress */
-	QUEUE_ORDSEQ_POSTFLUSH	= 0x10,	/* post-flushing in progress */
-	QUEUE_ORDSEQ_DONE	= 0x20,
+	QUEUE_HAS_FLUSH		= 1 << 0,	/* supports REQ_FLUSH */
+	QUEUE_HAS_FUA		= 1 << 1,	/* supports REQ_FUA */
 };
 
 #define blk_queue_plugged(q)	test_bit(QUEUE_FLAG_PLUGGED, &(q)->queue_flags)
@@ -879,11 +844,9 @@ extern void blk_queue_softirq_done(struc
 extern void blk_queue_rq_timed_out(struct request_queue *, rq_timed_out_fn *);
 extern void blk_queue_rq_timeout(struct request_queue *, unsigned int);
 extern struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev);
-extern int blk_queue_ordered(struct request_queue *, unsigned);
-extern bool blk_do_ordered(struct request_queue *, struct request **);
+extern int blk_queue_cache_features(struct request_queue *, unsigned);
 extern unsigned blk_ordered_cur_seq(struct request_queue *);
 extern unsigned blk_ordered_req_seq(struct request *);
-extern bool blk_ordered_complete_seq(struct request_queue *, unsigned, int);
 
 extern int blk_rq_map_sg(struct request_queue *, struct request *, struct scatterlist *);
 extern void blk_dump_rq_flags(struct request *, char *);
Index: linux-2.6/drivers/block/virtio_blk.c
===================================================================
--- linux-2.6.orig/drivers/block/virtio_blk.c	2010-08-07 12:53:23.800479189 -0400
+++ linux-2.6/drivers/block/virtio_blk.c	2010-08-07 14:51:34.198479189 -0400
@@ -388,31 +388,8 @@ static int __devinit virtblk_probe(struc
 	vblk->disk->driverfs_dev = &vdev->dev;
 	index++;
 
-	if (virtio_has_feature(vdev, VIRTIO_BLK_F_FLUSH)) {
-		/*
-		 * If the FLUSH feature is supported we do have support for
-		 * flushing a volatile write cache on the host.  Use that
-		 * to implement write barrier support.
-		 */
-		blk_queue_ordered(q, QUEUE_ORDERED_DRAIN_FLUSH);
-	} else if (virtio_has_feature(vdev, VIRTIO_BLK_F_BARRIER)) {
-		/*
-		 * If the BARRIER feature is supported the host expects us
-		 * to order request by tags.  This implies there is not
-		 * volatile write cache on the host, and that the host
-		 * never re-orders outstanding I/O.  This feature is not
-		 * useful for real life scenarious and deprecated.
-		 */
-		blk_queue_ordered(q, QUEUE_ORDERED_TAG);
-	} else {
-		/*
-		 * If the FLUSH feature is not supported we must assume that
-		 * the host does not perform any kind of volatile write
-		 * caching. We still need to drain the queue to provider
-		 * proper barrier semantics.
-		 */
-		blk_queue_ordered(q, QUEUE_ORDERED_DRAIN);
-	}
+	if (virtio_has_feature(vdev, VIRTIO_BLK_F_FLUSH))
+		blk_queue_cache_features(q, QUEUE_HAS_FLUSH);
 
 	/* If disk is read-only in the host, the guest should obey */
 	if (virtio_has_feature(vdev, VIRTIO_BLK_F_RO))
Index: linux-2.6/drivers/scsi/sd.c
===================================================================
--- linux-2.6.orig/drivers/scsi/sd.c	2010-08-07 12:53:23.872479189 -0400
+++ linux-2.6/drivers/scsi/sd.c	2010-08-07 14:54:47.812479189 -0400
@@ -2109,7 +2109,7 @@ static int sd_revalidate_disk(struct gen
 	struct scsi_disk *sdkp = scsi_disk(disk);
 	struct scsi_device *sdp = sdkp->device;
 	unsigned char *buffer;
-	unsigned ordered;
+	unsigned ordered = 0;
 
 	SCSI_LOG_HLQUEUE(3, sd_printk(KERN_INFO, sdkp,
 				      "sd_revalidate_disk\n"));
@@ -2151,17 +2151,14 @@ static int sd_revalidate_disk(struct gen
 
 	/*
 	 * We now have all cache related info, determine how we deal
-	 * with ordered requests.  Note that as the current SCSI
-	 * dispatch function can alter request order, we cannot use
-	 * QUEUE_ORDERED_TAG_* even when ordered tag is supported.
+	 * with barriers.
 	 */
-	if (sdkp->WCE)
-		ordered = sdkp->DPOFUA
-			? QUEUE_ORDERED_DRAIN_FUA : QUEUE_ORDERED_DRAIN_FLUSH;
-	else
-		ordered = QUEUE_ORDERED_DRAIN;
-
-	blk_queue_ordered(sdkp->disk->queue, ordered);
+	if (sdkp->WCE) {
+		ordered |= QUEUE_HAS_FLUSH;
+		if (sdkp->DPOFUA)
+			ordered |= QUEUE_HAS_FUA;
+	}
+	blk_queue_cache_features(sdkp->disk->queue, ordered);
 
 	set_capacity(disk, sdkp->capacity);
 	kfree(buffer);
Index: linux-2.6/block/blk-core.c
===================================================================
--- linux-2.6.orig/block/blk-core.c	2010-08-07 12:53:23.744479189 -0400
+++ linux-2.6/block/blk-core.c	2010-08-07 14:56:35.087479189 -0400
@@ -520,6 +520,7 @@ struct request_queue *blk_alloc_queue_no
 	init_timer(&q->unplug_timer);
 	setup_timer(&q->timeout, blk_rq_timed_out_timer, (unsigned long) q);
 	INIT_LIST_HEAD(&q->timeout_list);
+	INIT_LIST_HEAD(&q->pending_barriers);
 	INIT_WORK(&q->unplug_work, blk_unplug_work);
 
 	kobject_init(&q->kobj, &blk_queue_ktype);
@@ -1037,22 +1038,6 @@ void blk_insert_request(struct request_q
 }
 EXPORT_SYMBOL(blk_insert_request);
 
-/*
- * add-request adds a request to the linked list.
- * queue lock is held and interrupts disabled, as we muck with the
- * request queue list.
- */
-static inline void add_request(struct request_queue *q, struct request *req)
-{
-	drive_stat_acct(req, 1);
-
-	/*
-	 * elevator indicated where it wants this request to be
-	 * inserted at elevator_merge time
-	 */
-	__elv_add_request(q, req, ELEVATOR_INSERT_SORT, 0);
-}
-
 static void part_round_stats_single(int cpu, struct hd_struct *part,
 				    unsigned long now)
 {
@@ -1201,13 +1186,9 @@ static int __make_request(struct request
 	const bool sync = (bio->bi_rw & REQ_SYNC);
 	const bool unplug = (bio->bi_rw & REQ_UNPLUG);
 	const unsigned int ff = bio->bi_rw & REQ_FAILFAST_MASK;
+	int where = ELEVATOR_INSERT_SORT;
 	int rw_flags;
 
-	if ((bio->bi_rw & REQ_HARDBARRIER) &&
-	    (q->next_ordered == QUEUE_ORDERED_NONE)) {
-		bio_endio(bio, -EOPNOTSUPP);
-		return 0;
-	}
 	/*
 	 * low level driver can indicate that it wants pages above a
 	 * certain limit bounced to low memory (ie for highmem, or even
@@ -1217,7 +1198,12 @@ static int __make_request(struct request
 
 	spin_lock_irq(q->queue_lock);
 
-	if (unlikely((bio->bi_rw & REQ_HARDBARRIER)) || elv_queue_empty(q))
+	if (bio->bi_rw & REQ_HARDBARRIER) {
+		where = ELEVATOR_INSERT_ORDERED;
+		goto get_rq;
+	}
+
+	if (elv_queue_empty(q))
 		goto get_rq;
 
 	el_ret = elv_merge(q, &req, bio);
@@ -1314,7 +1300,10 @@ get_rq:
 		req->cpu = blk_cpu_to_group(smp_processor_id());
 	if (queue_should_plug(q) && elv_queue_empty(q))
 		blk_plug_device(q);
-	add_request(q, req);
+
+	/* insert the request into the elevator */
+	drive_stat_acct(req, 1);
+	__elv_add_request(q, req, where, 0);
 out:
 	if (unplug || !queue_should_plug(q))
 		__generic_unplug_device(q);
Index: linux-2.6/block/elevator.c
===================================================================
--- linux-2.6.orig/block/elevator.c	2010-08-07 12:53:23.752479189 -0400
+++ linux-2.6/block/elevator.c	2010-08-07 12:53:53.162479190 -0400
@@ -564,7 +564,7 @@ void elv_requeue_request(struct request_
 
 	rq->cmd_flags &= ~REQ_STARTED;
 
-	elv_insert(q, rq, ELEVATOR_INSERT_REQUEUE);
+	elv_insert(q, rq, ELEVATOR_INSERT_FRONT);
 }
 
 void elv_drain_elevator(struct request_queue *q)
@@ -611,8 +611,6 @@ void elv_quiesce_end(struct request_queu
 
 void elv_insert(struct request_queue *q, struct request *rq, int where)
 {
-	struct list_head *pos;
-	unsigned ordseq;
 	int unplug_it = 1;
 
 	trace_block_rq_insert(q, rq);
@@ -622,10 +620,14 @@ void elv_insert(struct request_queue *q,
 	switch (where) {
 	case ELEVATOR_INSERT_FRONT:
 		rq->cmd_flags |= REQ_SOFTBARRIER;
-
 		list_add(&rq->queuelist, &q->queue_head);
 		break;
 
+	case ELEVATOR_INSERT_ORDERED:
+		rq->cmd_flags |= REQ_SOFTBARRIER;
+		list_add_tail(&rq->queuelist, &q->queue_head);
+		break;
+
 	case ELEVATOR_INSERT_BACK:
 		rq->cmd_flags |= REQ_SOFTBARRIER;
 		elv_drain_elevator(q);
@@ -662,36 +664,6 @@ void elv_insert(struct request_queue *q,
 		q->elevator->ops->elevator_add_req_fn(q, rq);
 		break;
 
-	case ELEVATOR_INSERT_REQUEUE:
-		/*
-		 * If ordered flush isn't in progress, we do front
-		 * insertion; otherwise, requests should be requeued
-		 * in ordseq order.
-		 */
-		rq->cmd_flags |= REQ_SOFTBARRIER;
-
-		/*
-		 * Most requeues happen because of a busy condition,
-		 * don't force unplug of the queue for that case.
-		 */
-		unplug_it = 0;
-
-		if (q->ordseq == 0) {
-			list_add(&rq->queuelist, &q->queue_head);
-			break;
-		}
-
-		ordseq = blk_ordered_req_seq(rq);
-
-		list_for_each(pos, &q->queue_head) {
-			struct request *pos_rq = list_entry_rq(pos);
-			if (ordseq <= blk_ordered_req_seq(pos_rq))
-				break;
-		}
-
-		list_add_tail(&rq->queuelist, pos);
-		break;
-
 	default:
 		printk(KERN_ERR "%s: bad insertion point %d\n",
 		       __func__, where);
@@ -710,33 +682,15 @@ void elv_insert(struct request_queue *q,
 void __elv_add_request(struct request_queue *q, struct request *rq, int where,
 		       int plug)
 {
-	if (q->ordcolor)
-		rq->cmd_flags |= REQ_ORDERED_COLOR;
-
 	if (rq->cmd_flags & (REQ_SOFTBARRIER | REQ_HARDBARRIER)) {
-		/*
-		 * toggle ordered color
-		 */
-		if (rq->cmd_flags & REQ_HARDBARRIER)
-			q->ordcolor ^= 1;
-
-		/*
-		 * barriers implicitly indicate back insertion
-		 */
-		if (where == ELEVATOR_INSERT_SORT)
-			where = ELEVATOR_INSERT_BACK;
-
-		/*
-		 * this request is scheduling boundary, update
-		 * end_sector
-		 */
+		/* barriers are scheduling boundary, update end_sector */
 		if (rq->cmd_type == REQ_TYPE_FS ||
 		    (rq->cmd_flags & REQ_DISCARD)) {
 			q->end_sector = rq_end_sector(rq);
 			q->boundary_rq = rq;
 		}
 	} else if (!(rq->cmd_flags & REQ_ELVPRIV) &&
-		    where == ELEVATOR_INSERT_SORT)
+		   where == ELEVATOR_INSERT_SORT)
 		where = ELEVATOR_INSERT_BACK;
 
 	if (plug)
@@ -849,24 +803,6 @@ void elv_completed_request(struct reques
 		    e->ops->elevator_completed_req_fn)
 			e->ops->elevator_completed_req_fn(q, rq);
 	}
-
-	/*
-	 * Check if the queue is waiting for fs requests to be
-	 * drained for flush sequence.
-	 */
-	if (unlikely(q->ordseq)) {
-		struct request *next = NULL;
-
-		if (!list_empty(&q->queue_head))
-			next = list_entry_rq(q->queue_head.next);
-
-		if (!queue_in_flight(q) &&
-		    blk_ordered_cur_seq(q) == QUEUE_ORDSEQ_DRAIN &&
-		    (!next || blk_ordered_req_seq(next) > QUEUE_ORDSEQ_DRAIN)) {
-			blk_ordered_complete_seq(q, QUEUE_ORDSEQ_DRAIN, 0);
-			__blk_run_queue(q);
-		}
-	}
 }
 
 #define to_elv(atr) container_of((atr), struct elv_fs_entry, attr)
Index: linux-2.6/block/blk.h
===================================================================
--- linux-2.6.orig/block/blk.h	2010-08-07 12:53:23.762479189 -0400
+++ linux-2.6/block/blk.h	2010-08-07 12:53:53.171479190 -0400
@@ -51,6 +51,8 @@ static inline void blk_clear_rq_complete
  */
 #define ELV_ON_HASH(rq)		(!hlist_unhashed(&(rq)->hash))
 
+struct request *blk_do_ordered(struct request_queue *q, struct request *rq);
+
 static inline struct request *__elv_next_request(struct request_queue *q)
 {
 	struct request *rq;
@@ -58,7 +60,8 @@ static inline struct request *__elv_next
 	while (1) {
 		while (!list_empty(&q->queue_head)) {
 			rq = list_entry_rq(q->queue_head.next);
-			if (blk_do_ordered(q, &rq))
+			rq = blk_do_ordered(q, rq);
+			if (rq)
 				return rq;
 		}
 
Index: linux-2.6/drivers/block/xen-blkfront.c
===================================================================
--- linux-2.6.orig/drivers/block/xen-blkfront.c	2010-08-07 12:53:23.807479189 -0400
+++ linux-2.6/drivers/block/xen-blkfront.c	2010-08-07 14:44:39.564479189 -0400
@@ -417,30 +417,6 @@ static int xlvbd_init_blk_queue(struct g
 	return 0;
 }
 
-
-static int xlvbd_barrier(struct blkfront_info *info)
-{
-	int err;
-	const char *barrier;
-
-	switch (info->feature_barrier) {
-	case QUEUE_ORDERED_DRAIN:	barrier = "enabled (drain)"; break;
-	case QUEUE_ORDERED_TAG:		barrier = "enabled (tag)"; break;
-	case QUEUE_ORDERED_NONE:	barrier = "disabled"; break;
-	default:			return -EINVAL;
-	}
-
-	err = blk_queue_ordered(info->rq, info->feature_barrier);
-
-	if (err)
-		return err;
-
-	printk(KERN_INFO "blkfront: %s: barriers %s\n",
-	       info->gd->disk_name, barrier);
-	return 0;
-}
-
-
 static int xlvbd_alloc_gendisk(blkif_sector_t capacity,
 			       struct blkfront_info *info,
 			       u16 vdisk_info, u16 sector_size)
@@ -516,8 +492,6 @@ static int xlvbd_alloc_gendisk(blkif_sec
 	info->rq = gd->queue;
 	info->gd = gd;
 
-	xlvbd_barrier(info);
-
 	if (vdisk_info & VDISK_READONLY)
 		set_disk_ro(gd, 1);
 
@@ -662,8 +636,6 @@ static irqreturn_t blkif_interrupt(int i
 				printk(KERN_WARNING "blkfront: %s: write barrier op failed\n",
 				       info->gd->disk_name);
 				error = -EOPNOTSUPP;
-				info->feature_barrier = QUEUE_ORDERED_NONE;
-				xlvbd_barrier(info);
 			}
 			/* fall through */
 		case BLKIF_OP_READ:
@@ -1073,24 +1045,6 @@ static void blkfront_connect(struct blkf
 			    "feature-barrier", "%lu", &barrier,
 			    NULL);
 
-	/*
-	 * If there's no "feature-barrier" defined, then it means
-	 * we're dealing with a very old backend which writes
-	 * synchronously; draining will do what needs to get done.
-	 *
-	 * If there are barriers, then we can do full queued writes
-	 * with tagged barriers.
-	 *
-	 * If barriers are not supported, then there's no much we can
-	 * do, so just set ordering to NONE.
-	 */
-	if (err)
-		info->feature_barrier = QUEUE_ORDERED_DRAIN;
-	else if (barrier)
-		info->feature_barrier = QUEUE_ORDERED_TAG;
-	else
-		info->feature_barrier = QUEUE_ORDERED_NONE;
-
 	err = xlvbd_alloc_gendisk(sectors, info, binfo, sector_size);
 	if (err) {
 		xenbus_dev_fatal(info->xbdev, err, "xlvbd_add at %s",
Index: linux-2.6/drivers/ide/ide-disk.c
===================================================================
--- linux-2.6.orig/drivers/ide/ide-disk.c	2010-08-07 12:53:23.889479189 -0400
+++ linux-2.6/drivers/ide/ide-disk.c	2010-08-07 15:00:30.215479189 -0400
@@ -518,12 +518,13 @@ static int ide_do_setfeature(ide_drive_t
 
 static void update_ordered(ide_drive_t *drive)
 {
-	u16 *id = drive->id;
-	unsigned ordered = QUEUE_ORDERED_NONE;
+	unsigned ordered = 0;
 
 	if (drive->dev_flags & IDE_DFLAG_WCACHE) {
+		u16 *id = drive->id;
 		unsigned long long capacity;
 		int barrier;
+
 		/*
 		 * We must avoid issuing commands a drive does not
 		 * understand or we may crash it. We check flush cache
@@ -543,13 +544,18 @@ static void update_ordered(ide_drive_t *
 		       drive->name, barrier ? "" : "not ");
 
 		if (barrier) {
-			ordered = QUEUE_ORDERED_DRAIN_FLUSH;
+			printk(KERN_INFO "%s: cache flushes supported\n",
+				drive->name);
 			blk_queue_prep_rq(drive->queue, idedisk_prep_fn);
+			ordered |= QUEUE_HAS_FLUSH;
+		} else {
+			printk(KERN_INFO
+				"%s: WARNING: cache flushes not supported\n",
+				drive->name);
 		}
-	} else
-		ordered = QUEUE_ORDERED_DRAIN;
+	}
 
-	blk_queue_ordered(drive->queue, ordered);
+	blk_queue_cache_features(drive->queue, ordered);
 }
 
 ide_devset_get_flag(wcache, IDE_DFLAG_WCACHE);
Index: linux-2.6/drivers/md/dm.c
===================================================================
--- linux-2.6.orig/drivers/md/dm.c	2010-08-07 12:53:23.905479189 -0400
+++ linux-2.6/drivers/md/dm.c	2010-08-07 14:51:38.240479189 -0400
@@ -1908,7 +1908,7 @@ static struct mapped_device *alloc_dev(i
 	blk_queue_softirq_done(md->queue, dm_softirq_done);
 	blk_queue_prep_rq(md->queue, dm_prep_fn);
 	blk_queue_lld_busy(md->queue, dm_lld_busy);
-	blk_queue_ordered(md->queue, QUEUE_ORDERED_DRAIN_FLUSH);
+	blk_queue_cache_features(md->queue, QUEUE_HAS_FLUSH);
 
 	md->disk = alloc_disk(1);
 	if (!md->disk)
Index: linux-2.6/drivers/mmc/card/queue.c
===================================================================
--- linux-2.6.orig/drivers/mmc/card/queue.c	2010-08-07 12:53:23.927479189 -0400
+++ linux-2.6/drivers/mmc/card/queue.c	2010-08-07 14:30:09.666479189 -0400
@@ -128,7 +128,6 @@ int mmc_init_queue(struct mmc_queue *mq,
 	mq->req = NULL;
 
 	blk_queue_prep_rq(mq->queue, mmc_prep_request);
-	blk_queue_ordered(mq->queue, QUEUE_ORDERED_DRAIN);
 	queue_flag_set_unlocked(QUEUE_FLAG_NONROT, mq->queue);
 
 #ifdef CONFIG_MMC_BLOCK_BOUNCE
Index: linux-2.6/drivers/s390/block/dasd.c
===================================================================
--- linux-2.6.orig/drivers/s390/block/dasd.c	2010-08-07 12:53:23.939479189 -0400
+++ linux-2.6/drivers/s390/block/dasd.c	2010-08-07 14:30:13.307479189 -0400
@@ -2197,7 +2197,6 @@ static void dasd_setup_queue(struct dasd
 	 */
 	blk_queue_max_segment_size(block->request_queue, PAGE_SIZE);
 	blk_queue_segment_boundary(block->request_queue, PAGE_SIZE - 1);
-	blk_queue_ordered(block->request_queue, QUEUE_ORDERED_DRAIN);
 }
 
 /*
Index: linux-2.6/include/linux/elevator.h
===================================================================
--- linux-2.6.orig/include/linux/elevator.h	2010-08-07 12:53:23.781479189 -0400
+++ linux-2.6/include/linux/elevator.h	2010-08-07 12:53:53.208479190 -0400
@@ -162,9 +162,9 @@ extern struct request *elv_rb_find(struc
  * Insertion selection
  */
 #define ELEVATOR_INSERT_FRONT	1
-#define ELEVATOR_INSERT_BACK	2
-#define ELEVATOR_INSERT_SORT	3
-#define ELEVATOR_INSERT_REQUEUE	4
+#define ELEVATOR_INSERT_ORDERED	2
+#define ELEVATOR_INSERT_BACK	3
+#define ELEVATOR_INSERT_SORT	4
 
 /*
  * return values from elevator_may_queue_fn
Index: linux-2.6/drivers/block/pktcdvd.c
===================================================================
--- linux-2.6.orig/drivers/block/pktcdvd.c	2010-08-07 12:53:23.815479189 -0400
+++ linux-2.6/drivers/block/pktcdvd.c	2010-08-07 12:53:53.211479190 -0400
@@ -753,7 +753,6 @@ static int pkt_generic_packet(struct pkt
 
 	rq->timeout = 60*HZ;
 	rq->cmd_type = REQ_TYPE_BLOCK_PC;
-	rq->cmd_flags |= REQ_HARDBARRIER;
 	if (cgc->quiet)
 		rq->cmd_flags |= REQ_QUIET;
 
Index: linux-2.6/drivers/block/brd.c
===================================================================
--- linux-2.6.orig/drivers/block/brd.c	2010-08-07 12:53:23.825479189 -0400
+++ linux-2.6/drivers/block/brd.c	2010-08-07 14:26:12.293479191 -0400
@@ -482,7 +482,6 @@ static struct brd_device *brd_alloc(int
 	if (!brd->brd_queue)
 		goto out_free_dev;
 	blk_queue_make_request(brd->brd_queue, brd_make_request);
-	blk_queue_ordered(brd->brd_queue, QUEUE_ORDERED_TAG);
 	blk_queue_max_hw_sectors(brd->brd_queue, 1024);
 	blk_queue_bounce_limit(brd->brd_queue, BLK_BOUNCE_ANY);
 
Index: linux-2.6/drivers/block/loop.c
===================================================================
--- linux-2.6.orig/drivers/block/loop.c	2010-08-07 12:53:23.836479189 -0400
+++ linux-2.6/drivers/block/loop.c	2010-08-07 14:51:27.937479189 -0400
@@ -831,8 +831,8 @@ static int loop_set_fd(struct loop_devic
 	lo->lo_queue->queuedata = lo;
 	lo->lo_queue->unplug_fn = loop_unplug;
 
-	if (!(lo_flags & LO_FLAGS_READ_ONLY) && file->f_op->fsync)
-		blk_queue_ordered(lo->lo_queue, QUEUE_ORDERED_DRAIN);
+	/* XXX(hch): loop can't properly deal with flush requests currently */
+//	blk_queue_cache_features(lo->lo_queue, QUEUE_HAS_FLUSH);
 
 	set_capacity(lo->lo_disk, size);
 	bd_set_size(bdev, size << 9);
Index: linux-2.6/drivers/block/osdblk.c
===================================================================
--- linux-2.6.orig/drivers/block/osdblk.c	2010-08-07 12:53:23.843479189 -0400
+++ linux-2.6/drivers/block/osdblk.c	2010-08-07 14:51:30.091479189 -0400
@@ -439,7 +439,7 @@ static int osdblk_init_disk(struct osdbl
 	blk_queue_stack_limits(q, osd_request_queue(osdev->osd));
 
 	blk_queue_prep_rq(q, blk_queue_start_tag);
-	blk_queue_ordered(q, QUEUE_ORDERED_DRAIN_FLUSH);
+	blk_queue_cache_features(q, QUEUE_HAS_FLUSH);
 
 	disk->queue = q;
 
Index: linux-2.6/drivers/block/ps3disk.c
===================================================================
--- linux-2.6.orig/drivers/block/ps3disk.c	2010-08-07 12:53:23.859479189 -0400
+++ linux-2.6/drivers/block/ps3disk.c	2010-08-07 14:51:32.204479189 -0400
@@ -468,7 +468,7 @@ static int __devinit ps3disk_probe(struc
 	blk_queue_dma_alignment(queue, dev->blk_size-1);
 	blk_queue_logical_block_size(queue, dev->blk_size);
 
-	blk_queue_ordered(queue, QUEUE_ORDERED_DRAIN_FLUSH);
+	blk_queue_cache_features(queue, QUEUE_HAS_FLUSH);
 
 	blk_queue_max_segments(queue, -1);
 	blk_queue_max_segment_size(queue, dev->bounce_size);
Index: linux-2.6/include/linux/blk_types.h
===================================================================
--- linux-2.6.orig/include/linux/blk_types.h	2010-08-07 12:53:23.793479189 -0400
+++ linux-2.6/include/linux/blk_types.h	2010-08-07 12:53:53.243479190 -0400
@@ -141,7 +141,6 @@ enum rq_flag_bits {
 	__REQ_FAILED,		/* set if the request failed */
 	__REQ_QUIET,		/* don't worry about errors */
 	__REQ_PREEMPT,		/* set for "ide_preempt" requests */
-	__REQ_ORDERED_COLOR,	/* is before or after barrier */
 	__REQ_ALLOCED,		/* request came from our alloc pool */
 	__REQ_COPY_USER,	/* contains copies of user pages */
 	__REQ_INTEGRITY,	/* integrity metadata has been remapped */
@@ -181,7 +180,6 @@ enum rq_flag_bits {
 #define REQ_FAILED		(1 << __REQ_FAILED)
 #define REQ_QUIET		(1 << __REQ_QUIET)
 #define REQ_PREEMPT		(1 << __REQ_PREEMPT)
-#define REQ_ORDERED_COLOR	(1 << __REQ_ORDERED_COLOR)
 #define REQ_ALLOCED		(1 << __REQ_ALLOCED)
 #define REQ_COPY_USER		(1 << __REQ_COPY_USER)
 #define REQ_INTEGRITY		(1 << __REQ_INTEGRITY)

Re: [PATCH REPOST RFC] relaxed barriers

[Tejun Heo]

On 08/08/2010 04:31 PM, Christoph Hellwig wrote:
> On Sat, Aug 07, 2010 at 12:13:06PM +0200, Tejun Heo wrote:
>> The patch was on top of v2.6.35 but was generated against dirty tree
>> and wouldn't apply cleanly.  Here's the proper one.
> 
> Here's an updated version:
> 
>  (a) ported to Jens' current block tree
>  (b) optimize barriers on devices not requiring flushes to be no-ops
>  (b) redo the blk_queue_ordered interface to just set QUEUE_HAS_FLUSH
>      and QUEUE_HAS_FUA flags.

Nice.  I'm working on a properly split patchset implementing
REQ_FLUSH/FUA based interface, which replaces REQ_HARDBARRIER.  Empty
request w/ REQ_FLUSH just flushes cache but has no other ordering
restrictions.  REQ_FLUSH + data means preflush + data write.  REQ_FUA
+ data means data would be committed to NV media on completion.
REQ_FLUSH + FUA + data means preflush + NV data write.  All FLUSH/FUA
requests w/ data are ordered only against each other.  I think I'll be
able to post in several days.

Thanks.

-- 
tejun

Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

[Mike Snitzer]

On Wed, Aug 04 2010 at 10:16pm -0400,
Jun'ichi Nomura <j-nomura@ce.jp.nec.com> wrote:

> Hi Christoph,
> 
> (08/04/10 17:54), Christoph Hellwig wrote:
> > On Wed, Aug 04, 2010 at 01:57:37PM +0900, Kiyoshi Ueda wrote:
> >>> -		if (unlikely(dm_rq_is_flush_request(rq))) {
> >>> +		if (rq->cmd_flags & REQ_FLUSH) {
> >>>  			BUG_ON(md->flush_request);
> >>>  			md->flush_request = rq;
> >>>  			blk_start_request(rq);
> >>
> >> Current request-based device-mapper's flush code depends on
> >> the block-layer's barrier behavior which dispatches only one request
> >> at a time when flush is needed.
> >> In other words, current request-based device-mapper can't handle
> >> other requests while a flush request is in progress.
> >>
> >> I'll take a look how I can fix the request-based device-mapper to
> >> cope with it.  I think it'll take time for carefull investigation.
> > 
> > Given that request based device mapper doesn't even look at the
> > block numbers from what I can see just removing any special casing
> > for REQ_FLUSH should probably do it.
> 
> Special casing is necessary because device-mapper may have to
> send multiple copies of REQ_FLUSH request to multiple
> targets, while normal request is just sent to single target.

Yes, request-based DM is meant to have all the same capabilities as
bio-based DM.  So in theory it should support multiple targets but in
practice it doesn't.  DM's multipath target is the only consumer of
request-based DM and it only ever clones a single flush request
(num_flush_requests = 1).

So why not remove all of request-based DM's barrier infrastructure and
simply rely on the revised block layer to sequence the FLUSH+WRITE
request for request-based DM?

Given that we do not have a request-based DM target that requires
cloning multiple FLUSH requests its unused code that is delaying DM
support for the new FLUSH+FUA work (NOTE: bio-based DM obviously still
needs work in this area).

Once we have a need for using request-based DM for something other than
multipath we can take a fresh look at implementing rq-based FLUSH+FUA.

Mike

p.s. I know how hard NEC worked on request-based DM's barrier support;
so I'm not suggesting this lightly.  For me it just seems like we're
carrying complexity in DM that hasn't ever been required.

Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

[Mike Snitzer]

On Thu, Aug 26 2010 at  6:50pm -0400,
Mike Snitzer <snitzer@redhat.com> wrote:

> Once we have a need for using request-based DM for something other than
> multipath we can take a fresh look at implementing rq-based FLUSH+FUA.
> 
> Mike
> 
> p.s. I know how hard NEC worked on request-based DM's barrier support;
> so I'm not suggesting this lightly.  For me it just seems like we're
> carrying complexity in DM that hasn't ever been required.

To be clear: the piece that I was saying wasn't required is the need to
for request-based DM to clone a FLUSH to send to multiple targets
(saying as much was just a confusing distraction.. please ignore that).

Anyway, my previous email's question still stands.

Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

[Jamie Lokier]

Mike Snitzer wrote:
> On Thu, Aug 26 2010 at  6:50pm -0400,
> Mike Snitzer <snitzer@redhat.com> wrote:
> 
> > Once we have a need for using request-based DM for something other than
> > multipath we can take a fresh look at implementing rq-based FLUSH+FUA.
> > 
> > Mike
> > 
> > p.s. I know how hard NEC worked on request-based DM's barrier support;
> > so I'm not suggesting this lightly.  For me it just seems like we're
> > carrying complexity in DM that hasn't ever been required.
> 
> To be clear: the piece that I was saying wasn't required is the need to
> for request-based DM to clone a FLUSH to send to multiple targets
> (saying as much was just a confusing distraction.. please ignore that).
> 
> Anyway, my previous email's question still stands.

On a slightly related note: DM suggests a reason for the lower layer, or the
request queues, to implement the trivial optimisation of discarding
FLUSHes if there's been no WRITE since the previous FLUSH.

That was mentioned elsewhere in this big thread as not being worth
even the small effort - because the filesystem is able to make good
decisions anyway.

But once you have something like RAID or striping, it's quite common
for the filesystem to issue a FLUSH when only a subset of the target
devices have received WRITEs through the RAID/striping layer since
they last received a FLUSH.

-- Jamie

Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

[Jun'ichi Nomura]

Hi Mike,

(08/27/10 07:50), Mike Snitzer wrote:
>> Special casing is necessary because device-mapper may have to
>> send multiple copies of REQ_FLUSH request to multiple
>> targets, while normal request is just sent to single target.
> 
> Yes, request-based DM is meant to have all the same capabilities as
> bio-based DM.  So in theory it should support multiple targets but in
> practice it doesn't.  DM's multipath target is the only consumer of
> request-based DM and it only ever clones a single flush request
> (num_flush_requests = 1).

This is correct. But,

> So why not remove all of request-based DM's barrier infrastructure and
> simply rely on the revised block layer to sequence the FLUSH+WRITE
> request for request-based DM?
> 
> Given that we do not have a request-based DM target that requires
> cloning multiple FLUSH requests its unused code that is delaying DM
> support for the new FLUSH+FUA work (NOTE: bio-based DM obviously still
> needs work in this area).

the above mentioned 'special casing' is not a hard part.
See the attached patch.

The hard part is discerning the error type for flush failure
as discussed in the other thread.
And as Kiyoshi wrote, that's an existing problem so it can
be worked on as a separate issue than the new FLUSH work.

Thanks,
-- 
Jun'ichi Nomura, NEC Corporation


Cope with new sequencing of flush requests in the block layer.

Request-based dm used to depend on the barrier sequencer in the block layer
in that, when a flush request is dispatched, there are no other requests
in-flight. So it reused md->pending counter for checking completion of
cloned flush requests.

This patch separates the pending counter for flush request
as a prepartion for the new FLUSH work, where a flush request can be
dispatched while other normal requests are in-flight.

Index: linux-2.6.36-rc2/drivers/md/dm.c
===================================================================
--- linux-2.6.36-rc2.orig/drivers/md/dm.c
+++ linux-2.6.36-rc2/drivers/md/dm.c
@@ -162,6 +162,7 @@ struct mapped_device {
 
 	/* A pointer to the currently processing pre/post flush request */
 	struct request *flush_request;
+	atomic_t flush_pending;
 
 	/*
 	 * The current mapping.
@@ -777,10 +778,16 @@ static void store_barrier_error(struct m
  * the md may be freed in dm_put() at the end of this function.
  * Or do dm_get() before calling this function and dm_put() later.
  */
-static void rq_completed(struct mapped_device *md, int rw, int run_queue)
+static void rq_completed(struct mapped_device *md, int rw, int run_queue, bool is_flush)
 {
 	atomic_dec(&md->pending[rw]);
 
+	if (is_flush) {
+		atomic_dec(&md->flush_pending);
+		if (!atomic_read(&md->flush_pending))
+			wake_up(&md->wait);
+	}
+
 	/* nudge anyone waiting on suspend queue */
 	if (!md_in_flight(md))
 		wake_up(&md->wait);
@@ -837,7 +844,7 @@ static void dm_end_request(struct reques
 	} else
 		blk_end_request_all(rq, error);
 
-	rq_completed(md, rw, run_queue);
+	rq_completed(md, rw, run_queue, is_barrier);
 }
 
 static void dm_unprep_request(struct request *rq)
@@ -880,7 +887,7 @@ void dm_requeue_unmapped_request(struct 
 	blk_requeue_request(q, rq);
 	spin_unlock_irqrestore(q->queue_lock, flags);
 
-	rq_completed(md, rw, 0);
+	rq_completed(md, rw, 0, false);
 }
 EXPORT_SYMBOL_GPL(dm_requeue_unmapped_request);
 
@@ -1993,6 +2000,7 @@ static struct mapped_device *alloc_dev(i
 
 	atomic_set(&md->pending[0], 0);
 	atomic_set(&md->pending[1], 0);
+	atomic_set(&md->flush_pending, 0);
 	init_waitqueue_head(&md->wait);
 	INIT_WORK(&md->work, dm_wq_work);
 	INIT_WORK(&md->barrier_work, dm_rq_barrier_work);
@@ -2375,7 +2383,7 @@ void dm_put(struct mapped_device *md)
 }
 EXPORT_SYMBOL_GPL(dm_put);
 
-static int dm_wait_for_completion(struct mapped_device *md, int interruptible)
+static int dm_wait_for_completion(struct mapped_device *md, int interruptible, bool for_flush)
 {
 	int r = 0;
 	DECLARE_WAITQUEUE(wait, current);
@@ -2388,6 +2396,8 @@ static int dm_wait_for_completion(struct
 		set_current_state(interruptible);
 
 		smp_mb();
+		if (for_flush && !atomic_read(&md->flush_pending))
+			break;
 		if (!md_in_flight(md))
 			break;
 
@@ -2408,14 +2418,14 @@ static int dm_wait_for_completion(struct
 
 static void dm_flush(struct mapped_device *md)
 {
-	dm_wait_for_completion(md, TASK_UNINTERRUPTIBLE);
+	dm_wait_for_completion(md, TASK_UNINTERRUPTIBLE, false);
 
 	bio_init(&md->barrier_bio);
 	md->barrier_bio.bi_bdev = md->bdev;
 	md->barrier_bio.bi_rw = WRITE_BARRIER;
 	__split_and_process_bio(md, &md->barrier_bio);
 
-	dm_wait_for_completion(md, TASK_UNINTERRUPTIBLE);
+	dm_wait_for_completion(md, TASK_UNINTERRUPTIBLE, false);
 }
 
 static void process_barrier(struct mapped_device *md, struct bio *bio)
@@ -2512,11 +2522,12 @@ static int dm_rq_barrier(struct mapped_d
 			clone = clone_rq(md->flush_request, md, GFP_NOIO);
 			dm_rq_set_target_request_nr(clone, j);
 			atomic_inc(&md->pending[rq_data_dir(clone)]);
+			atomic_inc(&md->flush_pending);
 			map_request(ti, clone, md);
 		}
 	}
 
-	dm_wait_for_completion(md, TASK_UNINTERRUPTIBLE);
+	dm_wait_for_completion(md, TASK_UNINTERRUPTIBLE, true);
 	dm_table_put(map);
 
 	return md->barrier_error;
@@ -2705,7 +2716,7 @@ int dm_suspend(struct mapped_device *md,
 	 * We call dm_wait_for_completion to wait for all existing requests
 	 * to finish.
 	 */
-	r = dm_wait_for_completion(md, TASK_INTERRUPTIBLE);
+	r = dm_wait_for_completion(md, TASK_INTERRUPTIBLE, false);
 
 	down_write(&md->io_lock);
 	if (noflush)

Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

[Mike Snitzer]

On Thu, Aug 26 2010 at  9:43pm -0400,
Jun'ichi Nomura <j-nomura@ce.jp.nec.com> wrote:

> Hi Mike,
> 
> (08/27/10 07:50), Mike Snitzer wrote:
> >> Special casing is necessary because device-mapper may have to
> >> send multiple copies of REQ_FLUSH request to multiple
> >> targets, while normal request is just sent to single target.
> > 
> > Yes, request-based DM is meant to have all the same capabilities as
> > bio-based DM.  So in theory it should support multiple targets but in
> > practice it doesn't.  DM's multipath target is the only consumer of
> > request-based DM and it only ever clones a single flush request
> > (num_flush_requests = 1).
> 
> This is correct. But,
> 
> > So why not remove all of request-based DM's barrier infrastructure and
> > simply rely on the revised block layer to sequence the FLUSH+WRITE
> > request for request-based DM?
> > 
> > Given that we do not have a request-based DM target that requires
> > cloning multiple FLUSH requests its unused code that is delaying DM
> > support for the new FLUSH+FUA work (NOTE: bio-based DM obviously still
> > needs work in this area).
> 
> the above mentioned 'special casing' is not a hard part.
> See the attached patch.

Yes, Tejun suggested something like this in one of the threads.  Thanks
for implementing it.

But do you agree that the request-based barrier code (added in commit
d0bcb8786) could be reverted given the new FLUSH work?

We no longer need waiting now that ordering isn't a concern.  Especially
so given rq-based doesn't support multiple targets.  As you know, from
dm_table_set_type:

        /*
         * Request-based dm supports only tables that have a single target now.
         * To support multiple targets, request splitting support is needed,
         * and that needs lots of changes in the block-layer.
         * (e.g. request completion process for partial completion.)
         */

I think we need to at least benchmark the performance of dm-mpath
without any of this extra, soon to be unnecessary, code.

Maybe my concern is overblown...

> The hard part is discerning the error type for flush failure
> as discussed in the other thread.
> And as Kiyoshi wrote, that's an existing problem so it can
> be worked on as a separate issue than the new FLUSH work.

Right, Mike Christie will be refreshing his patchset that should enable
us to resolve that separate issue.

Thanks,
Mike

Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

[Jun'ichi Nomura]

Hi Mike,

(08/27/10 13:08), Mike Snitzer wrote:
>> the above mentioned 'special casing' is not a hard part.
>> See the attached patch.
> 
> Yes, Tejun suggested something like this in one of the threads.  Thanks
> for implementing it.
> 
> But do you agree that the request-based barrier code (added in commit
> d0bcb8786) could be reverted given the new FLUSH work?

No, it's a separate thing.
If we don't need to care about the case where multiple clones
of flush request are necessary, the special casing of flush
request can be removed regardless of the new FLUSH work.

> We no longer need waiting now that ordering isn't a concern.  Especially

The waiting is not for ordering, but for multiple clones.

> so given rq-based doesn't support multiple targets.  As you know, from
> dm_table_set_type:
> 
>         /*
>          * Request-based dm supports only tables that have a single target now.
>          * To support multiple targets, request splitting support is needed,
>          * and that needs lots of changes in the block-layer.
>          * (e.g. request completion process for partial completion.)
>          */

This comment is about multiple targets.
The special code for barrier is for single target whose
num_flush_requests > 1. Different thing.

> I think we need to at least benchmark the performance of dm-mpath
> without any of this extra, soon to be unnecessary, code.

If there will be no need for supporting a request-based target
with num_flush_requests > 1, the special handling of flush
can be removed.

And since there is no such target in the current tree,
I don't object if you remove that part of code for good reason.

Thanks,
-- 
Jun'ichi Nomura, NEC Corporation

Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

[Mike Snitzer]

On Fri, Aug 27 2010 at  1:52am -0400,
Jun'ichi Nomura <j-nomura@ce.jp.nec.com> wrote:

> Hi Mike,
> 
> (08/27/10 13:08), Mike Snitzer wrote:
> > But do you agree that the request-based barrier code (added in commit
> > d0bcb8786) could be reverted given the new FLUSH work?
> 
> No, it's a separate thing.
> If we don't need to care about the case where multiple clones
> of flush request are necessary, the special casing of flush
> request can be removed regardless of the new FLUSH work.

Ah, yes thanks for clarifying.  But we've never cared about multiple
clone of a flush so it's odd that such elaborate infrastructure was
introduced without a need.

> > We no longer need waiting now that ordering isn't a concern.  Especially
> 
> The waiting is not for ordering, but for multiple clones.
> 
> > so given rq-based doesn't support multiple targets.  As you know, from
> > dm_table_set_type:
> > 
> >         /*
> >          * Request-based dm supports only tables that have a single target now.
> >          * To support multiple targets, request splitting support is needed,
> >          * and that needs lots of changes in the block-layer.
> >          * (e.g. request completion process for partial completion.)
> >          */
> 
> This comment is about multiple targets.
> The special code for barrier is for single target whose
> num_flush_requests > 1. Different thing.

Yes, I need to not send mail just before going to bed..
 
> > I think we need to at least benchmark the performance of dm-mpath
> > without any of this extra, soon to be unnecessary, code.
> 
> If there will be no need for supporting a request-based target
> with num_flush_requests > 1, the special handling of flush
> can be removed.
> 
> And since there is no such target in the current tree,
> I don't object if you remove that part of code for good reason.

OK, certainly something to keep in mind.  But _really_ knowing the
multipath FLUSH+FUA performance difference (extra special-case code vs
none) requires a full FLUSH conversion of request-based DM anyway.

In general, request-based DM's barrier/flush code does carry a certain
maintenance overhead.  It is quite a bit of distracting code in the core
DM which isn't buying us anything.. so we _could_ just remove it and
never look back (until we have some specific need for num_flush_requests
> 1 in rq-based DM).

Mike

Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

[Jun'ichi Nomura]

Hi Mike,

(08/27/10 23:13), Mike Snitzer wrote:
>> If there will be no need for supporting a request-based target
>> with num_flush_requests > 1, the special handling of flush
>> can be removed.
>>
>> And since there is no such target in the current tree,
>> I don't object if you remove that part of code for good reason.
> 
> OK, certainly something to keep in mind.  But _really_ knowing the
> multipath FLUSH+FUA performance difference (extra special-case code vs
> none) requires a full FLUSH conversion of request-based DM anyway.
> 
> In general, request-based DM's barrier/flush code does carry a certain
> maintenance overhead.  It is quite a bit of distracting code in the core
> DM which isn't buying us anything.. so we _could_ just remove it and
> never look back (until we have some specific need for num_flush_requests
>> 1 in rq-based DM).

So, I'm not objecting to your idea.
Could you please create a patch to remove that?

Thanks,
-- 
Jun'ichi Nomura, NEC Corporation

Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

[Tejun Heo]

On 08/30/2010 06:45 AM, Jun'ichi Nomura wrote:
> Hi Mike,
> 
> (08/27/10 23:13), Mike Snitzer wrote:
>>> If there will be no need for supporting a request-based target
>>> with num_flush_requests > 1, the special handling of flush
>>> can be removed.
>>>
>>> And since there is no such target in the current tree,
>>> I don't object if you remove that part of code for good reason.
>>
>> OK, certainly something to keep in mind.  But _really_ knowing the
>> multipath FLUSH+FUA performance difference (extra special-case code vs
>> none) requires a full FLUSH conversion of request-based DM anyway.
>>
>> In general, request-based DM's barrier/flush code does carry a certain
>> maintenance overhead.  It is quite a bit of distracting code in the core
>> DM which isn't buying us anything.. so we _could_ just remove it and
>> never look back (until we have some specific need for num_flush_requests
>>> 1 in rq-based DM).
> 
> So, I'm not objecting to your idea.
> Could you please create a patch to remove that?

I did that yesterday.  Will post the patch soon.

Thanks.

-- 
tejun

Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

[Mike Snitzer]

On Mon, Aug 30 2010 at  4:33am -0400,
Tejun Heo <tj@kernel.org> wrote:

> On 08/30/2010 06:45 AM, Jun'ichi Nomura wrote:
> > Hi Mike,
> > 
> > (08/27/10 23:13), Mike Snitzer wrote:
> >>> If there will be no need for supporting a request-based target
> >>> with num_flush_requests > 1, the special handling of flush
> >>> can be removed.
> >>>
> >>> And since there is no such target in the current tree,
> >>> I don't object if you remove that part of code for good reason.
> >>
> >> OK, certainly something to keep in mind.  But _really_ knowing the
> >> multipath FLUSH+FUA performance difference (extra special-case code vs
> >> none) requires a full FLUSH conversion of request-based DM anyway.
> >>
> >> In general, request-based DM's barrier/flush code does carry a certain
> >> maintenance overhead.  It is quite a bit of distracting code in the core
> >> DM which isn't buying us anything.. so we _could_ just remove it and
> >> never look back (until we have some specific need for num_flush_requests
> >>> 1 in rq-based DM).
> > 
> > So, I'm not objecting to your idea.
> > Could you please create a patch to remove that?
> 
> I did that yesterday.  Will post the patch soon.

I did it yesterday also, mine builds on your previous DM patchset...

I'll review your recent patchset, from today, to compare and will share
my findings.

I was hoping we could get the current request-based code working with
your new FLUSH+FUA work without removing support for num_flush_requests
(yet).  And then layer in the removal to give us the before and after so
we would know the overhead associated with keeping/dropping
num_flush_requests.  But like I said earlier "we _could_ just remove it
and never look back".

Thanks,
Mike

Re: [PATCH, RFC 2/2] dm: support REQ_FLUSH directly

[Tejun Heo]

Hello,

On 08/30/2010 02:43 PM, Mike Snitzer wrote:
> I did it yesterday also, mine builds on your previous DM patchset...
> 
> I'll review your recent patchset, from today, to compare and will share
> my findings.

Thanks. :-)

> I was hoping we could get the current request-based code working with
> your new FLUSH+FUA work without removing support for num_flush_requests
> (yet).  And then layer in the removal to give us the before and after so
> we would know the overhead associated with keeping/dropping
> num_flush_requests.  But like I said earlier "we _could_ just remove it
> and never look back".

I tried but it's not very easy because the original implementation
depended on the block layer suppressing other requests while flush
sequence is in progress.  The painful part was that block layer no
longer sorts requeued flush requests in front of other front inserted
requests, so explicit queue suppressing can't be implemented simply.
Another route would be adding a separate wait/wakeup logic for flushes
(someone posted a demo patch for that which was almost there but not
fully), but it seemed like a aimless effort to build a new facility to
rip it out in the next patch.  After all, the whole thing seemed
somewhat pointless given that writes can't be routed to multiple
targets (if writes can't target multiple devices, flushes won't need
to either).

Thanks.

-- 
tejun