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

* [PATCH V2] blk-mq: don't grab rq's refcount in blk_mq_check_expired()
@ 2021-08-11 15:52 Ming Lei
  2021-08-12  8:55 ` Christoph Hellwig
                   ` (3 more replies)
  0 siblings, 4 replies; 5+ messages in thread
From: Ming Lei @ 2021-08-11 15:52 UTC (permalink / raw)
  To: Jens Axboe
  Cc: linux-block, Christoph Hellwig, Bart Van Assche, John Garry,
	Ming Lei, Keith Busch

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 <ming.lei@redhat.com>
---
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


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