block: fix zone write plugs refcount handling in disk_zone_wplug_schedule_bio_work()

The function disk_zone_wplug_schedule_bio_work() always takes a
reference on the zone write plug of the BIO work being scheduled. This
ensures 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 released 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 sure to drop the reference taken when scheduling the BIO work if
the work is already scheduled, that is, when queue_work() returns false.

Fixes: 9e78c38 ("block: Hold a reference on zone write plugs to schedule submission")
Cc: [email protected]
Signed-off-by: Damien Le Moal <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Bart Van Assche <[email protected]>
Reviewed-by: Johannes Thumshirn <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

Author: damien-lemoal

block/blk-zoned.c

@@ -1154,13 +1154,17 @@ 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. Taking a reference
+	 * to the zone write plug is required as the bio_work belongs to the
+	 * plug, and thus we must ensure that the write plug 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,
+	 * and we also drop this reference if the work is already scheduled.
 	 */
 	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))
+		disk_put_zone_wplug(zwplug);
 }
 
 static inline void disk_zone_wplug_add_bio(struct gendisk *disk,