path: root/block
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authorJens Axboe <jens.axboe@oracle.com>2007-04-25 11:53:48 +0200
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2007-04-25 08:41:48 -0700
commit5044eed48886b105a123333fe7ca97c6bd496120 (patch)
tree76233c2b177d9be75d3e1278b89ea5d3f7d87fcf /block
parenta23cf14b161b8deeb0f701d577a0e8be6365e247 (diff)
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>
Diffstat (limited to 'block')
1 files changed, 6 insertions, 6 deletions
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 9e3797167c8..f92ba2a869b 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -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
@@ -1639,12 +1645,6 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *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);
- /*
* we never wait for an async request and we don't allow preemption
* of an async request. so just return early