[PATCH v2 2/2] blk-mq: Fix a race condition in blk_mq_mark_tag_wait()

[Bart Van Assche <bart.vanassche@wdc.com>]

Both add_wait_queue() and blk_mq_dispatch_wake() protect wait queue
manipulations with the wait queue lock. Hence also protect the
!list_empty(&wait->entry) test with the wait queue lock instead of
the hctx lock.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Hannes Reinecke <hare@suse.de>
Cc: Johannes Thumshirn <jthumshirn@suse.de>
---
 block/blk-mq.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index e770e8814f60..d5313ce60836 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1184,7 +1184,7 @@ static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx **hctx,
 	bool shared_tags = (this_hctx->flags & BLK_MQ_F_TAG_SHARED) != 0;
 	struct sbq_wait_state *ws;
 	wait_queue_entry_t *wait;
-	bool ret;
+	bool on_wait_list, ret;
 
 	if (!shared_tags) {
 		if (!test_bit(BLK_MQ_S_SCHED_RESTART, &this_hctx->state))
@@ -1204,13 +1204,15 @@ static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx **hctx,
 		if (!list_empty_careful(&wait->entry))
 			return false;
 
-		spin_lock(&this_hctx->lock);
-		if (!list_empty(&wait->entry)) {
-			spin_unlock(&this_hctx->lock);
+		ws = bt_wait_ptr(&this_hctx->tags->bitmap_tags, this_hctx);
+
+		spin_lock_irq(&ws->wait.lock);
+		on_wait_list = !list_empty(&wait->entry);
+		spin_unlock_irq(&ws->wait.lock);
+
+		if (on_wait_list)
 			return false;
-		}
 
-		ws = bt_wait_ptr(&this_hctx->tags->bitmap_tags, this_hctx);
 		add_wait_queue(&ws->wait, wait);
 		/*
 		 * It's possible that a tag was freed in the window between the
@@ -1218,10 +1220,8 @@ static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx **hctx,
 		 * queue.
 		 */
 		ret = blk_mq_get_driver_tag(rq, hctx, false);
-		if (!ret) {
-			spin_unlock(&this_hctx->lock);
+		if (!ret)
 			return false;
-		}
 
 		/*
 		 * We got a tag, remove ourselves from the wait queue to ensure
@@ -1230,7 +1230,6 @@ static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx **hctx,
 		spin_lock_irq(&ws->wait.lock);
 		list_del_init(&wait->entry);
 		spin_unlock_irq(&ws->wait.lock);
-		spin_unlock(&this_hctx->lock);
 	}
 	return ret;
 }
-- 
2.15.1