[PATCH] blk_queue_start_tag() shared map race fix

If we share the tag map between two or more queues, then we cannot
use __set_bit() to set the bit. In fact we need to make sure we
atomically acquire this tag, so loop using test_and_set_bit() to
protect from that.

Noticed by Mike Christie <michaelc@cs.wisc.edu>

Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 9 insertions, 4 deletions

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index f757ed413214..83425fb3c8db 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -1171,11 +1171,16 @@ int blk_queue_start_tag(request_queue_t *q, struct request *rq)
                 BUG();
         }
 
-        tag = find_first_zero_bit(bqt->tag_map, bqt->max_depth);
-        if (tag >= bqt->max_depth)
-                return 1;
+        /*
+         * Protect against shared tag maps, as we may not have exclusive
+         * access to the tag map.
+         */
+        do {
+                tag = find_first_zero_bit(bqt->tag_map, bqt->max_depth);
+                if (tag >= bqt->max_depth)
+                        return 1;
 
-        __set_bit(tag, bqt->tag_map);
+        } while (test_and_set_bit(tag, bqt->tag_map));
 
         rq->cmd_flags |= REQ_QUEUED;
         rq->tag = tag;