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SubjectRe: INFO: task hung in blk_queue_enter
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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 \
--
Jens Axboe

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