Commit 5044eed4 authored Apr 25, 2007 by Jens Axboe Committed by Linus Torvalds Apr 25, 2007

cfq-iosched: fix alias + front merge bug

There's a really rare and obscure bug in CFQ, that causes a crash in
cfq_dispatch_insert() due to rq == NULL.  One example of the resulting
oops is seen here:

	http://lkml.org/lkml/2007/4/15/41

Neil correctly diagnosed the situation for how this can happen: if two
concurrent requests with the exact same sector number (due to direct IO
or aliasing between MD and the raw device access), the alias handling
will add the request to the sortlist, but next_rq remains NULL.

Read the more complete analysis at:

	http://lkml.org/lkml/2007/4/25/57



This looks like it requires md to trigger, even though it should
potentially be possible to due with O_DIRECT (at least if you edit the
kernel and doctor some of the unplug calls).

The fix is to move the ->next_rq update to when we add a request to the
rbtree. Then we remove the possibility for a request to exist in the
rbtree code, but not have ->next_rq correctly updated.

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

parent a23cf14b

block/cfq-iosched.c

+6 −6

Original line number	Diff line number	Diff line
		@@ -532,6 +532,12 @@ static void cfq_add_rq_rb(struct request *rq)

		if (!cfq_cfqq_on_rr(cfqq))
		cfq_add_cfqq_rr(cfqd, cfqq);

		/*
		* check if this request is a better next-serve candidate
		*/
		cfqq->next_rq = cfq_choose_req(cfqd, cfqq->next_rq, rq);
		BUG_ON(!cfqq->next_rq);
		}

		static inline void
		@@ -1638,12 +1644,6 @@ cfq_rq_enqueued(struct cfq_data cfqd, struct cfq_queue cfqq,
		if (rq_is_meta(rq))
		cfqq->meta_pending++;

		/*
		* check if this request is a better next-serve candidate)) {
		*/
		cfqq->next_rq = cfq_choose_req(cfqd, cfqq->next_rq, rq);
		BUG_ON(!cfqq->next_rq);

		/*
		* we never wait for an async request and we don't allow preemption
		* of an async request. so just return early

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