On Mon, Sep 24, 2018 at 12:51:07PM -0700, Bart Van Assche wrote:

On Mon, 2018-09-24 at 13:13 -0600, Keith Busch wrote:

quoted

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 85a1c1a59c72..28d128450621 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -848,22 +848,6 @@ static void blk_mq_timeout_work(struct work_struct *work)

	struct blk_mq_hw_ctx *hctx;
	int i;
 
-	/* A deadlock might occur if a request is stuck requiring a
-	 * timeout at the same time a queue freeze is waiting
-	 * completion, since the timeout code would not be able to
-	 * acquire the queue reference here.
-	 *
-	 * That's why we don't use blk_queue_enter here; instead, we use
-	 * percpu_ref_tryget directly, because we need to be able to
-	 * obtain a reference even in the short window between the queue
-	 * starting to freeze, by dropping the first reference in
-	 * blk_freeze_queue_start, and the moment the last request is
-	 * consumed, marked by the instant q_usage_counter reaches
-	 * zero.
-	 */
-	if (!percpu_ref_tryget(&q->q_usage_counter))
-		return;
-
	blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &next);
 
	if (next != 0) {

@@ -881,7 +865,6 @@ static void blk_mq_timeout_work(struct work_struct *work)

				blk_mq_tag_idle(hctx);
		}
	}
-	blk_queue_exit(q);
 }

Hi Keith,

The above introduces a behavior change: if the percpu_ref_tryget() call inside
blk_mq_queue_tag_busy_iter() fails then blk_mq_timeout_work() will now call
blk_mq_tag_idle(). I think that's wrong if the percpu_ref_tryget() call fails
due to the queue having been frozen. Please make blk_mq_queue_tag_busy_iter()
return a bool that indicates whether or not it has iterated over the request
queue.

Good point, thanks for the feedback.