[PATCH] blk-mq: move blk_mq_update_nr_hw_queues synchronize_rcu call

[PATCH] blk-mq: move blk_mq_update_nr_hw_queues synchronize_rcu call

[Giuliano Procida]

This fixes the
4.9 backport commit f530afb974c2e82047bd6220303a2dbe30eff304
which was
upstream commit f5bbbbe4d63577026f908a809f22f5fd5a90ea1f.

The upstream commit added a call to synchronize_rcu to
_blk_mq_update_nr_hw_queues, just after freezing queues.

In the backport this landed (in blk_mq_update_nr_hw_queues instead),
just after unfreezeing queues.

This commit moves the call to its intended place.

Fixes: f530afb974c2 ("blk-mq: sync the update nr_hw_queues with blk_mq_queue_tag_busy_iter")
Signed-off-by: Giuliano Procida <gprocida@google.com>
---
 block/blk-mq.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 58be2eaa5aaa..e0ed7317e98c 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2331,6 +2331,10 @@ void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int nr_hw_queues)
 
 	list_for_each_entry(q, &set->tag_list, tag_set_list)
 		blk_mq_freeze_queue(q);
+	/*
+	 * Sync with blk_mq_queue_tag_busy_iter.
+	 */
+	synchronize_rcu();
 
 	set->nr_hw_queues = nr_hw_queues;
 	list_for_each_entry(q, &set->tag_list, tag_set_list) {
@@ -2346,10 +2350,6 @@ void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int nr_hw_queues)
 
 	list_for_each_entry(q, &set->tag_list, tag_set_list)
 		blk_mq_unfreeze_queue(q);
-	/*
-	 * Sync with blk_mq_queue_tag_busy_iter.
-	 */
-	synchronize_rcu();
 }
 EXPORT_SYMBOL_GPL(blk_mq_update_nr_hw_queues);
 
-- 
2.27.0.290.gba653c62da-goog

Re: [PATCH] blk-mq: move blk_mq_update_nr_hw_queues synchronize_rcu call

[Sasha Levin]

On Thu, Jun 18, 2020 at 07:30:22PM +0100, Giuliano Procida wrote:
>This fixes the
>4.9 backport commit f530afb974c2e82047bd6220303a2dbe30eff304
>which was
>upstream commit f5bbbbe4d63577026f908a809f22f5fd5a90ea1f.
>
>The upstream commit added a call to synchronize_rcu to
>_blk_mq_update_nr_hw_queues, just after freezing queues.
>
>In the backport this landed (in blk_mq_update_nr_hw_queues instead),
>just after unfreezeing queues.
>
>This commit moves the call to its intended place.
>
>Fixes: f530afb974c2 ("blk-mq: sync the update nr_hw_queues with blk_mq_queue_tag_busy_iter")
>Signed-off-by: Giuliano Procida <gprocida@google.com>

Good catch! I've queued this and the 4.14 version.

-- 
Thanks,
Sasha