Re: [PATCH v3 4/4] block: block new I/O just after queue is set as dying

Before commit 780db2071a(blk-mq: decouble blk-mq freezing
from generic bypassing), the dying flag is checked before
entering queue, and Tejun converts the checking into .mq_freeze_depth,
and assumes the counter is increased just after dying flag
is set. Unfortunately we doesn't do that in blk_set_queue_dying().
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This patch calls blk_freeze_queue_start() in blk_set_queue_dying(),
so that we can block new I/O coming once the queue is set as dying.
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Given blk_set_queue_dying() is always called in remove path
of block device, and queue will be cleaned up later, we don't
need to worry about undoing the counter.
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Cc: Bart Van Assche <redacted>
Cc: Tejun Heo <tj@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
---
 block/blk-core.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)
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diff --git a/block/blk-core.c b/block/blk-core.c
index 60f364e1d36b..e22c4ea002ec 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c

@@ -500,6 +500,13 @@ void blk_set_queue_dying(struct request_queue *q)
 	queue_flag_set(QUEUE_FLAG_DYING, q);
 	spin_unlock_irq(q->queue_lock);

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+	/*
+	 * When queue DYING flag is set, we need to block new req
+	 * entering queue, so we call blk_freeze_queue_start() to
+	 * prevent I/O from crossing blk_queue_enter().
+	 */
+	blk_freeze_queue_start(q);

	if (q->mq_ops)
 		blk_mq_wake_waiters(q);
 	else {

@@ -672,9 +679,9 @@ int blk_queue_enter(struct request_queue *q, bool now=