[PATCH] blk-mq: fix blk_mq_hw_ctx active request accounting

[Tian Lan <tilan7663@gmail.com>]

From: Tian Lan <tian.lan@twosigma.com>

The nr_active counter continues to increase over time which causes the
blk_mq_get_tag to hang until the thread is rescheduled to a different
core despite there are still tags available.

kernel-stack

  INFO: task inboundIOReacto:3014879 blocked for more than 2 seconds
  Not tainted 6.1.15-amd64 #1 Debian 6.1.15~debian11
  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
  task:inboundIORe state:D stack:0  pid:3014879 ppid:4557 flags:0x00000000
    Call Trace:
    <TASK>
    __schedule+0x351/0xa20
    scheduler+0x5d/0xe0
    io_schedule+0x42/0x70
    blk_mq_get_tag+0x11a/0x2a0
    ? dequeue_task_stop+0x70/0x70
    __blk_mq_alloc_requests+0x191/0x2e0

kprobe output showing RQF_MQ_INFLIGHT bit is not cleared before
__blk_mq_free_request being called.

  320    320  kworker/29:1H __blk_mq_free_request rq_flags 0x220c0
         b'__blk_mq_free_request+0x1 [kernel]'
         b'bt_iter+0x50 [kernel]'
         b'blk_mq_queue_tag_busy_iter+0x318 [kernel]'
         b'blk_mq_timeout_work+0x7c [kernel]'
         b'process_one_work+0x1c4 [kernel]'
         b'worker_thread+0x4d [kernel]'
         b'kthread+0xe6 [kernel]'
         b'ret_from_fork+0x1f [kernel]'

The issue is caused by the difference between blk_mq_free_request() and
blk_mq_end_request_batch() wrt. when to call __blk_mq_dec_active_requests().
The former does it before calling req_ref_put_and_test(), and the latter
decreases the active request after req_ref_put_and_test().

- Fixes: f794f3351f26 ("block: add support for blk_mq_end_request_batch()")

Signed-off-by: Tian Lan <tian.lan@twosigma.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index f6dad0886a2f..850bfb844ed2 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -683,6 +683,10 @@ static void __blk_mq_free_request(struct request *rq)
 	blk_crypto_free_request(rq);
 	blk_pm_mark_last_busy(rq);
 	rq->mq_hctx = NULL;
+
+	if (rq->rq_flags & RQF_MQ_INFLIGHT)
+		__blk_mq_dec_active_requests(hctx);
+
 	if (rq->tag != BLK_MQ_NO_TAG)
 		blk_mq_put_tag(hctx->tags, ctx, rq->tag);
 	if (sched_tag != BLK_MQ_NO_TAG)
@@ -694,15 +698,11 @@ static void __blk_mq_free_request(struct request *rq)
 void blk_mq_free_request(struct request *rq)
 {
 	struct request_queue *q = rq->q;
-	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
 
 	if ((rq->rq_flags & RQF_ELVPRIV) &&
 	    q->elevator->type->ops.finish_request)
 		q->elevator->type->ops.finish_request(rq);
 
-	if (rq->rq_flags & RQF_MQ_INFLIGHT)
-		__blk_mq_dec_active_requests(hctx);
-
 	if (unlikely(laptop_mode && !blk_rq_is_passthrough(rq)))
 		laptop_io_completion(q->disk->bdi);
 
-- 
2.25.1