[PATCH 4.18 229/235] blk-mq: avoid to synchronize rcu inside blk_cleanup_queue()

From: Greg Kroah-Hartman
Date: Mon Sep 24 2018 - 08:45:18 EST

4.18-stable review patch. If anyone has any objections, please let me know.


From: Ming Lei <ming.lei@xxxxxxxxxx>

[ Upstream commit 1311326cf4755c7ffefd20f576144ecf46d9906b ]

SCSI probing may synchronously create and destroy a lot of request_queues
for non-existent devices. Any synchronize_rcu() in queue creation or
destroy path may introduce long latency during booting, see detailed
description in comment of blk_register_queue().

This patch removes one synchronize_rcu() inside blk_cleanup_queue()
for this case, commit c2856ae2f315d75(blk-mq: quiesce queue before freeing queue)
needs synchronize_rcu() for implementing blk_mq_quiesce_queue(), but
when queue isn't initialized, it isn't necessary to do that since
only pass-through requests are involved, no original issue in
scsi_execute() at all.

Without this patch and previous one, it may take more 20+ seconds for
virtio-scsi to complete disk probe. With the two patches, the time becomes
less than 100ms.

Fixes: c2856ae2f315d75 ("blk-mq: quiesce queue before freeing queue")
Reported-by: Andrew Jones <drjones@xxxxxxxxxx>
Cc: Omar Sandoval <osandov@xxxxxx>
Cc: Bart Van Assche <bart.vanassche@xxxxxxx>
Cc: linux-scsi@xxxxxxxxxxxxxxx
Cc: "Martin K. Petersen" <martin.petersen@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Tested-by: Andrew Jones <drjones@xxxxxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Sasha Levin <alexander.levin@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
block/blk-core.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -791,9 +791,13 @@ void blk_cleanup_queue(struct request_qu
* make sure all in-progress dispatch are completed because
* blk_freeze_queue() can only complete all requests, and
* dispatch may still be in-progress since we dispatch requests
- * from more than one contexts
+ * from more than one contexts.
+ *
+ * No need to quiesce queue if it isn't initialized yet since
+ * blk_freeze_queue() should be enough for cases of passthrough
+ * request.
- if (q->mq_ops)
+ if (q->mq_ops && blk_queue_init_done(q))

/* for synchronous bio-based driver finish in-flight integrity i/o */