[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>
diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index f757ed4..83425fb 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -1171,11 +1171,16 @@
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;