path: root/block
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authorNick Piggin <npiggin@suse.de>2007-09-14 08:41:12 +0200
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2007-09-14 13:56:47 -0700
commitdd941252a81b02b5915e2db160fe02c972875846 (patch)
tree83414a197b2d831a9df3a11e31d277a1dfa4ae68 /block
parent19299b1a722198830e39264a0f2edadd3fde74c2 (diff)
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> --
Diffstat (limited to 'block')
1 files changed, 10 insertions, 0 deletions
diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index cd20367061d..ed39313c408 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;