block: change the tag sync vs async restriction logic

Make them fully share the tag space, but disallow async requests using
the last any two slots.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 9 insertions, 6 deletions

diff --git a/block/blk-tag.c b/block/blk-tag.c
index c260f7c30dd..2e5cfeb5933 100644
--- a/block/blk-tag.c
+++ b/block/blk-tag.c
@@ -336,7 +336,7 @@ 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;
+        unsigned max_depth;
         int tag;
 
         if (unlikely((rq->cmd_flags & REQ_QUEUED))) {
@@ -355,13 +355,16 @@ int blk_queue_start_tag(struct request_queue *q, struct request *rq)
          * 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;
+        if (!rq_is_sync(rq) && max_depth > 1) {
+                max_depth -= 2;
+                if (!max_depth)
+                        max_depth = 1;
+                if (q->in_flight[0] > max_depth)
+                        return 1;
+        }
 
         do {
-                tag = find_next_zero_bit(bqt->tag_map, max_depth, offset);
+                tag = find_first_zero_bit(bqt->tag_map, max_depth);
                 if (tag >= max_depth)
                         return 1;