Re: INFO: task hung in blk_queue_enter

Ming Lei
2018-06-02T02:36:30Z

On Fri, Jun 01, 2018 at 06:49:10PM -0600, Jens Axboe wrote:
> On 6/1/18 5:49 PM, Ming Lei wrote:
> > On Fri, Jun 01, 2018 at 11:52:45AM -0600, Jens Axboe wrote:
> >> On 6/1/18 4:10 AM, Tetsuo Handa wrote:
> >>> Tetsuo Handa wrote:
> >>>> Since sum of percpu_count did not change after percpu_ref_kill(), this is
> >>>> not a race condition while folding percpu counter values into atomic counter
> >>>> value. That is, for some reason, someone who is responsible for calling
> >>>> percpu_ref_put(&q->q_usage_counter) (presumably via blk_queue_exit()) is
> >>>> unable to call percpu_ref_put().
> >>>> But I don't know how to find someone who is failing to call percpu_ref_put()...
> >>>
> >>> I found the someone. It was already there in the backtrace...
> >>>
> >>> ----------------------------------------
> >>> [ 62.065852] a.out D 0 4414 4337 0x00000000
> >>> [ 62.067677] Call Trace:
> >>> [ 62.068545] __schedule+0x40b/0x860
> >>> [ 62.069726] schedule+0x31/0x80
> >>> [ 62.070796] schedule_timeout+0x1c1/0x3c0
> >>> [ 62.072159] ? __next_timer_interrupt+0xd0/0xd0
> >>> [ 62.073670] blk_queue_enter+0x218/0x520
> >>> [ 62.074985] ? remove_wait_queue+0x70/0x70
> >>> [ 62.076361] generic_make_request+0x3d/0x540
> >>> [ 62.077785] ? __bio_clone_fast+0x6b/0x80
> >>> [ 62.079147] ? bio_clone_fast+0x2c/0x70
> >>> [ 62.080456] blk_queue_split+0x29b/0x560
> >>> [ 62.081772] ? blk_queue_split+0x29b/0x560
> >>> [ 62.083162] blk_mq_make_request+0x7c/0x430
> >>> [ 62.084562] generic_make_request+0x276/0x540
> >>> [ 62.086034] submit_bio+0x6e/0x140
> >>> [ 62.087185] ? submit_bio+0x6e/0x140
> >>> [ 62.088384] ? guard_bio_eod+0x9d/0x1d0
> >>> [ 62.089681] do_mpage_readpage+0x328/0x730
> >>> [ 62.091045] ? __add_to_page_cache_locked+0x12e/0x1a0
> >>> [ 62.092726] mpage_readpages+0x120/0x190
> >>> [ 62.094034] ? check_disk_change+0x70/0x70
> >>> [ 62.095454] ? check_disk_change+0x70/0x70
> >>> [ 62.096849] ? alloc_pages_current+0x65/0xd0
> >>> [ 62.098277] blkdev_readpages+0x18/0x20
> >>> [ 62.099568] __do_page_cache_readahead+0x298/0x360
> >>> [ 62.101157] ondemand_readahead+0x1f6/0x490
> >>> [ 62.102546] ? ondemand_readahead+0x1f6/0x490
> >>> [ 62.103995] page_cache_sync_readahead+0x29/0x40
> >>> [ 62.105539] generic_file_read_iter+0x7d0/0x9d0
> >>> [ 62.107067] ? futex_wait+0x221/0x240
> >>> [ 62.108303] ? trace_hardirqs_on+0xd/0x10
> >>> [ 62.109654] blkdev_read_iter+0x30/0x40
> >>> [ 62.110954] generic_file_splice_read+0xc5/0x140
> >>> [ 62.112538] do_splice_to+0x74/0x90
> >>> [ 62.113726] splice_direct_to_actor+0xa4/0x1f0
> >>> [ 62.115209] ? generic_pipe_buf_nosteal+0x10/0x10
> >>> [ 62.116773] do_splice_direct+0x8a/0xb0
> >>> [ 62.118056] do_sendfile+0x1aa/0x390
> >>> [ 62.119255] __x64_sys_sendfile64+0x4e/0xc0
> >>> [ 62.120666] do_syscall_64+0x6e/0x210
> >>> [ 62.121909] entry_SYSCALL_64_after_hwframe+0x49/0xbe
> >>> ----------------------------------------
> >>>
> >>> The someone is blk_queue_split() from blk_mq_make_request() who depends on an
> >>> assumption that blk_queue_enter() from recursively called generic_make_request()
> >>> does not get blocked due to percpu_ref_tryget_live(&q->q_usage_counter) failure.
> >>>
> >>> ----------------------------------------
> >>> generic_make_request(struct bio *bio) {
> >>> if (blk_queue_enter(q, flags) < 0) { /* <= percpu_ref_tryget_live() succeeds. */
> >>> if (!blk_queue_dying(q) && (bio->bi_opf & REQ_NOWAIT))
> >>> bio_wouldblock_error(bio);
> >>> else
> >>> bio_io_error(bio);
> >>> return ret;
> >>> }
> >>> (...snipped...)
> >>> ret = q->make_request_fn(q, bio);
> >>> (...snipped...)
> >>> if (q)
> >>> blk_queue_exit(q);
> >>> }
> >>> ----------------------------------------
> >>>
> >>> where q->make_request_fn == blk_mq_make_request which does
> >>>
> >>> ----------------------------------------
> >>> blk_mq_make_request(struct request_queue *q, struct bio *bio) {
> >>> blk_queue_split(q, &bio);
> >>> }
> >>>
> >>> blk_queue_split(struct request_queue *q, struct bio **bio) {
> >>> generic_make_request(*bio); /* <= percpu_ref_tryget_live() fails and waits until atomic_read(&q->mq_freeze_depth) becomes 0. */
> >>> }
> >>> ----------------------------------------
> >>>
> >>> and meanwhile atomic_inc_return(&q->mq_freeze_depth) and
> >>> percpu_ref_kill() are called by blk_freeze_queue_start()...
> >>>
> >>> Now, it is up to you about how to fix this race problem.
> >>
> >> Ahh, nicely spotted! One idea would be the one below. For this case,
> >> we're recursing, so we can either do a non-block queue enter, or we
> >> can just do a live enter.
> >>
> >> diff --git a/block/blk-core.c b/block/blk-core.c
> >> index 85909b431eb0..7de12e0dcc3d 100644
> >> --- a/block/blk-core.c
> >> +++ b/block/blk-core.c
> >> @@ -2392,7 +2392,9 @@ blk_qc_t generic_make_request(struct bio *bio)
> >>
> >> if (bio->bi_opf & REQ_NOWAIT)
> >> flags = BLK_MQ_REQ_NOWAIT;
> >> - if (blk_queue_enter(q, flags) < 0) {
> >> + if (bio->bi_opf & REQ_QUEUE_ENTERED)
> >> + blk_queue_enter_live(q);
> >> + else if (blk_queue_enter(q, flags) < 0) {
> >> if (!blk_queue_dying(q) && (bio->bi_opf & REQ_NOWAIT))
> >> bio_wouldblock_error(bio);
> >> else
> >> diff --git a/block/blk-merge.c b/block/blk-merge.c
> >> index 782940c65d8a..e1063e8f7eda 100644
> >> --- a/block/blk-merge.c
> >> +++ b/block/blk-merge.c
> >> @@ -210,6 +210,16 @@ void blk_queue_split(struct request_queue *q, struct bio **bio)
> >> /* there isn't chance to merge the splitted bio */
> >> split->bi_opf |= REQ_NOMERGE;
> >>
> >> + /*
> >> + * Since we're recursing into make_request here, ensure
> >> + * that we mark this bio as already having entered the queue.
> >> + * If not, and the queue is going away, we can get stuck
> >> + * forever on waiting for the queue reference to drop. But
> >> + * that will never happen, as we're already holding a
> >> + * reference to it.
> >> + */
> >> + (*bio)->bi_opf |= REQ_QUEUE_ENTERED;
> >> +
> >> bio_chain(split, *bio);
> >> trace_block_split(q, split, (*bio)->bi_iter.bi_sector);
> >> generic_make_request(*bio);
> >> diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
> >> index 17b18b91ebac..a6bc789b83e7 100644
> >> --- a/include/linux/blk_types.h
> >> +++ b/include/linux/blk_types.h
> >> @@ -282,6 +282,8 @@ enum req_flag_bits {
> >> /* command specific flags for REQ_OP_WRITE_ZEROES: */
> >> __REQ_NOUNMAP, /* do not free blocks when zeroing */
> >>
> >> + __REQ_QUEUE_ENTERED, /* queue already entered for this request */
> >> +
> >> /* for driver use */
> >> __REQ_DRV,
> >>
> >> @@ -302,9 +304,8 @@ enum req_flag_bits {
> >> #define REQ_RAHEAD (1ULL << __REQ_RAHEAD)
> >> #define REQ_BACKGROUND (1ULL << __REQ_BACKGROUND)
> >> #define REQ_NOWAIT (1ULL << __REQ_NOWAIT)
> >> -
> >> #define REQ_NOUNMAP (1ULL << __REQ_NOUNMAP)
> >> -
> >> +#define REQ_QUEUE_ENTERED (1ULL << __REQ_QUEUE_ENTERED)
> >> #define REQ_DRV (1ULL << __REQ_DRV)
> >>
> >> #define REQ_FAILFAST_MASK \
> >
> > bio clone inherits .bi_opf, so this way may not work for cloned bio.
> >
> > Given most of bio_split() is run in .make_request_fn(), holding the
> > queue ref may not be needed in case of bio splitting, so another
> > candidate fix might be to introduce one extra parameter of
> > 'need_queue_ref' to generic_make_request().
>
> I did consider that, but that's pretty silly for a corner case and you'd
> have to change a crap ton of calls to it. I think it'd be cleaner to

Seems not necessary to change any call to it, since all blk_queue_split()
are called from q->make_request_fn() now.

> clear the bit when we need to, potentially even adding a debug check to
> blk_queue_enter_live() that complains if the ref was not already
> elevated. Though that would be expensive, compared to the percpu inc
> now. Not saying the bit is necessarily the best way forward, but I do
> like it a LOT more than adding an argument to generic_make_request.

We may introduce generic_make_request_with_ref() for blk_queue_split(),
what do you think of the following patch?

diff --git a/block/blk-core.c b/block/blk-core.c
index cee03cad99f2..e10ee3f9f338 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2363,7 +2363,7 @@ generic_make_request_checks(struct bio *bio)
* a lower device by calling into generic_make_request recursively, which
* means the bio should NOT be touched after the call to ->make_request_fn.
*/
-blk_qc_t generic_make_request(struct bio *bio)
+blk_qc_t __generic_make_request(struct bio *bio, bool got_ref)
{
/*
* bio_list_on_stack[0] contains bios submitted by the current
@@ -2379,12 +2379,14 @@ blk_qc_t generic_make_request(struct bio *bio)

if (bio->bi_opf & REQ_NOWAIT)
flags = BLK_MQ_REQ_NOWAIT;
- if (blk_queue_enter(q, flags) < 0) {
- if (!blk_queue_dying(q) && (bio->bi_opf & REQ_NOWAIT))
- bio_wouldblock_error(bio);
- else
- bio_io_error(bio);
- return ret;
+ if (!got_ref) {
+ if (blk_queue_enter(q, flags) < 0) {
+ if (!blk_queue_dying(q) && (bio->bi_opf & REQ_NOWAIT))
+ bio_wouldblock_error(bio);
+ else
+ bio_io_error(bio);
+ return ret;
+ }
}

if (!generic_make_request_checks(bio))
@@ -2426,8 +2428,9 @@ blk_qc_t generic_make_request(struct bio *bio)
bool enter_succeeded = true;

if (unlikely(q != bio->bi_disk->queue)) {
- if (q)
+ if (q && !got_ref)
blk_queue_exit(q);
+ got_ref = false;
q = bio->bi_disk->queue;
flags = 0;
if (bio->bi_opf & REQ_NOWAIT)
@@ -2476,7 +2479,7 @@ blk_qc_t generic_make_request(struct bio *bio)
blk_queue_exit(q);
return ret;
}
-EXPORT_SYMBOL(generic_make_request);
+EXPORT_SYMBOL(__generic_make_request);

/**
* direct_make_request - hand a buffer directly to its device driver for I/O
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 5573d0fbec53..997616f1c85e 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -180,6 +180,9 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
return do_split ? new : NULL;
}

+/*
+ * This function has to be called from q->make_request_fn().
+ */
void blk_queue_split(struct request_queue *q, struct bio **bio)
{
struct bio *split, *res;
@@ -212,7 +215,7 @@ void blk_queue_split(struct request_queue *q, struct bio **bio)

bio_chain(split, *bio);
trace_block_split(q, split, (*bio)->bi_iter.bi_sector);
- generic_make_request(*bio);
+ generic_make_request_with_ref(*bio);
*bio = split;
}
}
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 4efd9af62e25..051018e0126e 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -956,9 +956,18 @@ static inline void rq_flush_dcache_pages(struct request *rq)
}
#endif

+blk_qc_t generic_make_request(struct bio *bio)
+{
+ return __generic_make_request(bio, false);
+}
+
+blk_qc_t generic_make_request_with_ref(struct bio *bio)
+{
+ return __generic_make_request(bio, true);
+}
+
extern int blk_register_queue(struct gendisk *disk);
extern void blk_unregister_queue(struct gendisk *disk);
-extern blk_qc_t generic_make_request(struct bio *bio);
extern blk_qc_t direct_make_request(struct bio *bio);
extern void blk_rq_init(struct request_queue *q, struct request *rq);
extern void blk_init_request_from_bio(struct request *req, struct bio *bio);
>
> It should have been a bio flag to begin with, that better captures the
> scope and also avoids clones inheriting anything they shouldn't. It's
> also not applicable to a request.
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 85909b431eb0..8f1063f50863 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -2392,7 +2392,9 @@ blk_qc_t generic_make_request(struct bio *bio)
>
> if (bio->bi_opf & REQ_NOWAIT)
> flags = BLK_MQ_REQ_NOWAIT;
> - if (blk_queue_enter(q, flags) < 0) {
> + if (bio->bi_flags & BIO_QUEUE_ENTERED)
> + blk_queue_enter_live(q);
> + else if (blk_queue_enter(q, flags) < 0) {
> if (!blk_queue_dying(q) && (bio->bi_opf & REQ_NOWAIT))
> bio_wouldblock_error(bio);
> else
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 782940c65d8a..9ee9474e579c 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -210,6 +210,16 @@ void blk_queue_split(struct request_queue *q, struct bio **bio)
> /* there isn't chance to merge the splitted bio */
> split->bi_opf |= REQ_NOMERGE;
>
> + /*
> + * Since we're recursing into make_request here, ensure
> + * that we mark this bio as already having entered the queue.
> + * If not, and the queue is going away, we can get stuck
> + * forever on waiting for the queue reference to drop. But
> + * that will never happen, as we're already holding a
> + * reference to it.
> + */
> + (*bio)->bi_flags |= BIO_QUEUE_ENTERED;
> +
> bio_chain(split, *bio);
> trace_block_split(q, split, (*bio)->bi_iter.bi_sector);
> generic_make_request(*bio);
> diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
> index 17b18b91ebac..4687d7c99076 100644
> --- a/include/linux/blk_types.h
> +++ b/include/linux/blk_types.h
> @@ -186,6 +186,8 @@ struct bio {
> * throttling rules. Don't do it again. */
> #define BIO_TRACE_COMPLETION 10 /* bio_endio() should trace the final completion
> * of this bio. */
> +#define BIO_QUEUE_ENTERED 11 /* can use blk_queue_enter_live() */
> +
> /* See BVEC_POOL_OFFSET below before adding new flags */

In DM or MD, there is the following use case:

1) bio comes from fs, then one clone is made from this bio

2) the cloned bio is submitted via generic_make_request() from
another context, such as wq.

3) then the flag of BIO_QUEUE_ENTERED is inherited and causes
the queue usage counter isn't held.

Thanks,
Ming