Re: [PATCH] as i/o hang with aacraid driver 2.6.0-test1

[Jens Axboe <axboe@suse.de>]

On Wed, Jul 16 2003, James Bottomley wrote:
> > SCSI does it right, don't worry. It's a block layer problem that we need
> > to find out how to correctly make sure that we deal it. Basically
> > elv_next_request() does an implicit elv_started_request(), at least that
> > is what AS assumes. So request re-adds should notify the scheduler of
> > that fact.
> 
> What about exporting a more generic requeue function (perhaps one that
> would take care of ending the tag)?
> 
> Our current requeue code looks like
> 
> 	if (blk_rq_tagged(req))
> 		blk_queue_end_tag(q, req);
> 	__elv_add_request(q, req, 0, 0);
> 
> which looks slightly fragile given that we have to remember to end the
> tag and then call a __ function (which are usually private).

Not usually private, but lockless.

> If we called an explicit request requeue hook, schedulers that need to
> adjust for requeueing would have a single place they could plug into?

blk_insert_request() is close, but it does a little more (it's useful
another place in the scsi queueing, though). How about something like
this? Then I can add the 'back on queue but not completed' stuff after
that.

===== drivers/block/ll_rw_blk.c 1.192 vs edited =====
--- 1.192/drivers/block/ll_rw_blk.c	Sun Jul 13 14:15:43 2003
+++ edited/drivers/block/ll_rw_blk.c	Wed Jul 16 15:17:23 2003
@@ -1494,6 +1494,23 @@
 
 	return rq;
 }
+/**
+ * blk_requeue_request - put a request back on queue
+ * @q:		request queue where request should be inserted
+ * @rq:		request to be inserted
+ *
+ * Description:
+ *    Drivers often keep queueing requests until the hardware cannot accept
+ *    more, when that condition happens we need to put the request back
+ *    on the queue. Must be called with queue lock held.
+ */
+void blk_requeue_request(request_queue_t *q, struct request *rq)
+{
+	if (blk_rq_tagged(rq))
+		blk_queue_end_tag(q, rq);
+
+	__elv_add_request(q, rq, 0, 0);
+}
 
 /**
  * blk_insert_request - insert a special request in to a request queue
===== drivers/scsi/scsi_lib.c 1.99 vs edited =====
--- 1.99/drivers/scsi/scsi_lib.c	Mon Jun 30 03:14:44 2003
+++ edited/drivers/scsi/scsi_lib.c	Wed Jul 16 15:17:52 2003
@@ -444,22 +444,8 @@
  */
 static void scsi_requeue_command(struct request_queue *q, struct scsi_cmnd *cmd)
 {
-	unsigned long flags;
-
-	spin_lock_irqsave(q->queue_lock, flags);
-	cmd->request->special = cmd;
-	if (blk_rq_tagged(cmd->request))
-		blk_queue_end_tag(q, cmd->request);
-
-	/*
-	 * set REQ_SPECIAL - we have a command
-	 * clear REQ_DONTPREP - we assume the sg table has been 
-	 *	nuked so we need to set it up again.
-	 */
-	cmd->request->flags |= REQ_SPECIAL;
 	cmd->request->flags &= ~REQ_DONTPREP;
-	__elv_add_request(q, cmd->request, 0, 0);
-	spin_unlock_irqrestore(q->queue_lock, flags);
+	blk_insert_request(cmd->request, 1, cmd);
 
 	scsi_run_queue(q);
 }
@@ -1213,9 +1199,7 @@
 	 * later time.
 	 */
 	spin_lock_irq(q->queue_lock);
-	if (blk_rq_tagged(req))
-		blk_queue_end_tag(q, req);
-	__elv_add_request(q, req, 0, 0);
+	blk_requeue_request(q, req);
 	sdev->device_busy--;
 	if(sdev->device_busy == 0)
 		blk_plug_device(q);
===== include/linux/blkdev.h 1.116 vs edited =====
--- 1.116/include/linux/blkdev.h	Sat Jul  5 08:52:51 2003
+++ edited/include/linux/blkdev.h	Wed Jul 16 15:18:06 2003
@@ -491,6 +491,7 @@
 extern struct request *blk_get_request(request_queue_t *, int, int);
 extern void blk_put_request(struct request *);
 extern void blk_insert_request(request_queue_t *, struct request *, int, void *);
+extern void blk_requeue_request(request_queue_t *, struct request *);
 extern void blk_plug_device(request_queue_t *);
 extern int blk_remove_plug(request_queue_t *);
 extern void blk_recount_segments(request_queue_t *, struct bio *);

-- 
Jens Axboe