Commit f3da54ba authored Sep 13, 2007 by Jens Axboe Committed by Linus Torvalds Sep 13, 2007

Fix race with shared tag queue maps

There's a race condition in blk_queue_end_tag() for shared tag maps,
users include stex (promise supertrak thingy) and qla2xxx.  The former
at least has reported bugs in this area, not sure why we haven't seen
any for the latter.  It could be because the window is narrow and that
other conditions in the qla2xxx code hide this.  It's a real bug,
though, as the stex smp users can attest.

We need to ensure two things - the tag bit clearing needs to happen
AFTER we cleared the tag pointer, as the tag bit clearing/setting is
what protects this map.  Secondly, we need to ensure that the visibility
of the tag pointer and tag bit clear are ordered properly.

[ I removed the SMP barriers - "test_and_clear_bit()" already implies
  all the required barriers.  -- Linus ]

Also see http://bugzilla.kernel.org/show_bug.cgi?id=7842



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

parent 9ca2152e

block/ll_rw_blk.c

+7 −6

Original line number	Diff line number	Diff line
		@@ -1075,12 +1075,6 @@ void blk_queue_end_tag(struct request_queue q, struct request rq)
		*/
		return;

		if (unlikely(!__test_and_clear_bit(tag, bqt->tag_map))) {
		printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
		__FUNCTION__, tag);
		return;
		}

		list_del_init(&rq->queuelist);
		rq->cmd_flags &= ~REQ_QUEUED;
		rq->tag = -1;
		@@ -1090,6 +1084,13 @@ void blk_queue_end_tag(struct request_queue q, struct request rq)
		__FUNCTION__, tag);

		bqt->tag_index[tag] = NULL;

		if (unlikely(!test_and_clear_bit(tag, bqt->tag_map))) {
		printk(KERN_ERR "%s: attempt to clear non-busy tag (%d)\n",
		__FUNCTION__, tag);
		return;
		}

		bqt->busy--;
		}

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