diff options
| author | Nick Piggin <npiggin@suse.de> | 2007-09-14 08:41:12 +0200 |
|---|---|---|
| committer | Linus Torvalds <torvalds@woody.linux-foundation.org> | 2007-09-14 13:56:47 -0700 |
| commit | dd941252a81b02b5915e2db160fe02c972875846 (patch) | |
| tree | 83414a197b2d831a9df3a11e31d277a1dfa4ae68 /block | |
| parent | 19299b1a722198830e39264a0f2edadd3fde74c2 (diff) | |
| download | lwn-dd941252a81b02b5915e2db160fe02c972875846.tar.gz lwn-dd941252a81b02b5915e2db160fe02c972875846.zip | |
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')
| -rw-r--r-- | block/ll_rw_blk.c | 10 |
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; |