[PATCH 4/8] block: improve disk_zone_wplug_schedule_bio_work()

[Damien Le Moal <dlemoal@kernel.org>]

The function disk_zone_wplug_schedule_bio_work() always takes a
reference on the zone write plug of the BIO work being scheduled. This
ensure that the zone write plug cannot be freed while the BIO work is
being scheduled but has not run yet. However, this unconditional
reference taking is fragile since the reference taken is realized by the
BIO work blk_zone_wplug_bio_work() function, which implies that there
always must be a 1:1 relation between the work being scheduled and the
work running.

Make this less fragile by taking the reference on the BIO work if and
only if the BIO work has not been already scheduled.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
---
 block/blk-zoned.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/block/blk-zoned.c b/block/blk-zoned.c
index c5c91f927a71..4b388ae1acaa 100644
--- a/block/blk-zoned.c
+++ b/block/blk-zoned.c
@@ -1205,13 +1205,15 @@ static void disk_zone_wplug_schedule_bio_work(struct gendisk *disk,
 	lockdep_assert_held(&zwplug->lock);
 
 	/*
-	 * Take a reference on the zone write plug and schedule the submission
-	 * of the next plugged BIO. blk_zone_wplug_bio_work() will release the
-	 * reference we take here.
+	 * Schedule the submission of the next plugged BIO. If the zone write
+	 * plug BIO work is not already scheduled, take a reference on the zone
+	 * write plug to ensure that it does not go away while the work is being
+	 * scheduled but has not run yet. blk_zone_wplug_bio_work() will release
+	 * the reference we take here.
 	 */
 	WARN_ON_ONCE(!(zwplug->flags & BLK_ZONE_WPLUG_PLUGGED));
-	refcount_inc(&zwplug->ref);
-	queue_work(disk->zone_wplugs_wq, &zwplug->bio_work);
+	if (queue_work(disk->zone_wplugs_wq, &zwplug->bio_work))
+		refcount_inc(&zwplug->ref);
 }
 
 static inline void disk_zone_wplug_add_bio(struct gendisk *disk,
-- 
2.53.0