[PATCH V2] blk-mq: don't grab rq's refcount in blk_mq_check_expired()

Inside blk_mq_queue_tag_busy_iter() we already grabbed request's
refcount before calling ->fn(), so needn't to grab it one more time
in blk_mq_check_expired().

Meantime remove extra request expire check in blk_mq_check_expired().

Cc: Keith Busch <kbusch@kernel.org>
Signed-off-by: Ming Lei <redacted>
---
V2:
	- remove extra request expire check as suggested by Keith
	- modify comment a bit

 block/blk-mq.c | 30 +++++-------------------------
 1 file changed, 5 insertions(+), 25 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index d2725f94491d..b5237211ccb7 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -923,34 +923,14 @@ static bool blk_mq_check_expired(struct blk_mq_hw_ctx *hctx,
 	unsigned long *next = priv;
 
 	/*
-	 * Just do a quick check if it is expired before locking the request in
-	 * so we're not unnecessarilly synchronizing across CPUs.
-	 */
-	if (!blk_mq_req_expired(rq, next))
-		return true;
-
-	/*
-	 * We have reason to believe the request may be expired. Take a
-	 * reference on the request to lock this request lifetime into its
-	 * currently allocated context to prevent it from being reallocated in
-	 * the event the completion by-passes this timeout handler.
-	 *
-	 * If the reference was already released, then the driver beat the
-	 * timeout handler to posting a natural completion.
-	 */
-	if (!refcount_inc_not_zero(&rq->ref))
-		return true;
-
-	/*
-	 * The request is now locked and cannot be reallocated underneath the
-	 * timeout handler's processing. Re-verify this exact request is truly
-	 * expired; if it is not expired, then the request was completed and
-	 * reallocated as a new request.
+	 * blk_mq_queue_tag_busy_iter() has locked the request, so it cannot
+	 * be reallocated underneath the timeout handler's processing, then
+	 * the expire check is reliable. If the request is not expired, then
+	 * it was completed and reallocated as a new request after returning
+	 * from blk_mq_check_expired().
 	 */
 	if (blk_mq_req_expired(rq, next))
 		blk_mq_rq_timed_out(rq, reserved);
-
-	blk_mq_put_rq_ref(rq);
 	return true;
 }
 

-- 
2.31.1

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

On 11/08/2021 16:52, Ming Lei wrote:

Inside blk_mq_queue_tag_busy_iter() we already grabbed request's
refcount before calling ->fn(), so needn't to grab it one more time
in blk_mq_check_expired().

Meantime remove extra request expire check in blk_mq_check_expired().

Cc: Keith Busch<kbusch@kernel.org>
Signed-off-by: Ming Lei<redacted>

Reviewed-by: John Garry <redacted>

On Wed, Aug 11, 2021 at 11:52:02PM +0800, Ming Lei wrote:

Inside blk_mq_queue_tag_busy_iter() we already grabbed request's
refcount before calling ->fn(), so needn't to grab it one more time
in blk_mq_check_expired().

Meantime remove extra request expire check in blk_mq_check_expired().

Cc: Keith Busch <kbusch@kernel.org>
Signed-off-by: Ming Lei <redacted>
---
V2:
	- remove extra request expire check as suggested by Keith
	- modify comment a bit

Hello Jens,

Ping...

Thanks,
Ming

On 8/11/21 9:52 AM, Ming Lei wrote:

Inside blk_mq_queue_tag_busy_iter() we already grabbed request's
refcount before calling ->fn(), so needn't to grab it one more time
in blk_mq_check_expired().

Meantime remove extra request expire check in blk_mq_check_expired().

Applied, thanks Ming.

-- 
Jens Axboe