X-Git-Url: http://git.monstr.eu/?a=blobdiff_plain;f=block%2Fblk-mq.c;fp=block%2Fblk-mq.c;h=a4c54c5895a1390f52b67045a494442b20b910c1;hb=5266caaf5660529e3da53004b8b7174cab6374ed;hp=aa9a05fdd023776d30f98485063b2c291100853d;hpb=b2da197565d7a2e8f357605a63971bf39597a9f0;p=linux-2.6-microblaze.git

diff --git a/block/blk-mq.c b/block/blk-mq.c
index aa9a05fdd023..a4c54c5895a1 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1852,6 +1852,22 @@ static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx *hctx,
 	wait->flags &= ~WQ_FLAG_EXCLUSIVE;
 	__add_wait_queue(wq, wait);
 
+	/*
+	 * Add one explicit barrier since blk_mq_get_driver_tag() may
+	 * not imply barrier in case of failure.
+	 *
+	 * Order adding us to wait queue and allocating driver tag.
+	 *
+	 * The pair is the one implied in sbitmap_queue_wake_up() which
+	 * orders clearing sbitmap tag bits and waitqueue_active() in
+	 * __sbitmap_queue_wake_up(), since waitqueue_active() is lockless
+	 *
+	 * Otherwise, re-order of adding wait queue and getting driver tag
+	 * may cause __sbitmap_queue_wake_up() to wake up nothing because
+	 * the waitqueue_active() may not observe us in wait queue.
+	 */
+	smp_mb();
+
 	/*
 	 * It's possible that a tag was freed in the window between the
 	 * allocation failure and adding the hardware queue to the wait