blk-mq: fix IO hang from sbitmap wakeup race

[linux-2.6-microblaze.git] / block / blk-mq.c

diff --git a/block/blk-mq.c b/block/blk-mq.c
index aa9a05f..a4c54c5 100644 (file)
--- 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