shared tag queue barrier comment

Should add some comments for the tag barriers (they won't be so important
if we can switch over to the explicit _lock bitops, but for now we should
make it clear).

Jens' original patch said a barrier after the test_and_clear_bit was also
required. I can't see why (and it would prevent the use of the _lock bitop).

Acked-by: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
--

1 files changed, 10 insertions, 0 deletions

diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index cd20367061d7..ed39313c4085 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -1085,6 +1085,12 @@ void blk_queue_end_tag(struct request_queue *q, struct request *rq)
 
         bqt->tag_index[tag] = NULL;
 
+        /*
+         * We use test_and_clear_bit's memory ordering properties here.
+         * The tag_map bit acts as a lock for tag_index[bit], so we need
+         * a barrer before clearing the bit (precisely: release semantics).
+         * Could use clear_bit_unlock when it is merged.
+         */
         if (unlikely(!test_and_clear_bit(tag, bqt->tag_map))) {
                 printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
                        __FUNCTION__, tag);
@@ -1137,6 +1143,10 @@ int blk_queue_start_tag(struct request_queue *q, struct request *rq)
                         return 1;
 
         } while (test_and_set_bit(tag, bqt->tag_map));
+        /*
+         * We rely on test_and_set_bit providing lock memory ordering semantics
+         * (could use test_and_set_bit_lock when it is merged).
+         */
 
         rq->cmd_flags |= REQ_QUEUED;
         rq->tag = tag;