1 files changed, 12 insertions, 2 deletions

diff --git a/block/blk-tag.c b/block/blk-tag.c
index 8a99688eb1b1..c0d419e84ce7 100644
--- a/block/blk-tag.c
+++ b/block/blk-tag.c
@@ -337,6 +337,7 @@ EXPORT_SYMBOL(blk_queue_end_tag);
 int blk_queue_start_tag(struct request_queue *q, struct request *rq)
 {
         struct blk_queue_tag *bqt = q->queue_tags;
+        unsigned max_depth, offset;
         int tag;
 
         if (unlikely((rq->cmd_flags & REQ_QUEUED))) {
@@ -350,10 +351,19 @@ int blk_queue_start_tag(struct request_queue *q, struct request *rq)
         /*
          * Protect against shared tag maps, as we may not have exclusive
          * access to the tag map.
+         *
+         * We reserve a few tags just for sync IO, since we don't want
+         * to starve sync IO on behalf of flooding async IO.
          */
+        max_depth = bqt->max_depth;
+        if (rq_is_sync(rq))
+                offset = 0;
+        else
+                offset = max_depth >> 2;
+
         do {
-                tag = find_first_zero_bit(bqt->tag_map, bqt->max_depth);
-                if (tag >= bqt->max_depth)
+                tag = find_next_zero_bit(bqt->tag_map, max_depth, offset);
+                if (tag >= max_depth)
                         return 1;
 
         } while (test_and_set_bit_lock(tag, bqt->tag_map));