1 files changed, 17 insertions, 6 deletions

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index a15845c164f2..ed39313c4085 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -1075,12 +1075,6 @@ void blk_queue_end_tag(struct request_queue *q, struct request *rq)
                  */
                 return;
 
-        if (unlikely(!__test_and_clear_bit(tag, bqt->tag_map))) {
-                printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
-                       __FUNCTION__, tag);
-                return;
-        }
-
         list_del_init(&rq->queuelist);
         rq->cmd_flags &= ~REQ_QUEUED;
         rq->tag = -1;
@@ -1090,6 +1084,19 @@ void blk_queue_end_tag(struct request_queue *q, struct request *rq)
                        __FUNCTION__, tag);
 
         bqt->tag_index[tag] = NULL;
+
+        /*
+         * We use test_and_clear_bit's memory ordering properties here.
+         * The tag_map bit acts as a lock for tag_index[bit], so we need
+         * a barrer before clearing the bit (precisely: release semantics).
+         * Could use clear_bit_unlock when it is merged.
+         */
+        if (unlikely(!test_and_clear_bit(tag, bqt->tag_map))) {
+                printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
+                       __FUNCTION__, tag);
+                return;
+        }
+
         bqt->busy--;
 }
 
@@ -1136,6 +1143,10 @@ int blk_queue_start_tag(struct request_queue *q, struct request *rq)
                         return 1;
 
         } while (test_and_set_bit(tag, bqt->tag_map));
+        /*
+         * We rely on test_and_set_bit providing lock memory ordering semantics
+         * (could use test_and_set_bit_lock when it is merged).
+         */
 
         rq->cmd_flags |= REQ_QUEUED;
         rq->tag = tag;