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-rw-r--r--block/blk-tag.c14
1 files changed, 12 insertions, 2 deletions
diff --git a/block/blk-tag.c b/block/blk-tag.c
index 8a99688eb1b..c0d419e84ce 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));