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shared tag queue barrier comment

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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>
--
This commit is contained in:
Nick Piggin
2007-09-14 08:41:12 +02:00
committed by Linus Torvalds
parent 19299b1a72
commit dd941252a8
1 changed files with 10 additions and 0 deletions
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10
block/ll_rw_blk.c
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@@ -1085,6 +1085,12 @@ void blk_queue_end_tag(struct request_queue *q, struct request *rq)
bqt->tag_index[tag] = NULL; 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))) { if (unlikely(!test_and_clear_bit(tag, bqt->tag_map))) {
printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n", printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
__FUNCTION__, tag); __FUNCTION__, tag);
@@ -1137,6 +1143,10 @@ int blk_queue_start_tag(struct request_queue *q, struct request *rq)
return 1; return 1;
} while (test_and_set_bit(tag, bqt->tag_map)); } 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->cmd_flags |= REQ_QUEUED;
rq->tag = tag; rq->tag = tag;