shared tag queue barrier comment (dd941252) · Commits · 戴 / test

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> --

block/ll_rw_blk.c

+10 −0

Original line number	Diff line number	Diff line
		@@ -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;

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