Re: PROBLEM: null pointer dereference in cfq_dispatch_requests (2.6.21-rc2 and 2.6.20)

[Johannes Weiner]

Hi,

I think I found where the NULL may come from.  Please, anybody, do not
apply this patch before a trustful person reviewed it... Jens? ;)

My thoughts on this are, that there are two possibilities cfqq->next_rq
could be NULL: End of list or a bug when it is set (or not set).
But why does RB_EMPTY_ROOT() as last call in this loop does not trigger?

Did I even get the right place on where the NULL pointer dereference
happens? :)

=Hannes

Signed-off-by: Johannes Weiner <hannes-kernel@xxxxxxxxxxxx>
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index b6491c0..ca84f0b 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -961,8 +961,8 @@ __cfq_dispatch_requests(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 		/*
 		 * follow expired path, else get first next available
 		 */
-		if ((rq = cfq_check_fifo(cfqq)) == NULL)
-			rq = cfqq->next_rq;
+		if (!(rq = cfq_check_fifo(cfqq)) && !(rq = cfqq->next_rq))
+			break;
 
 		/*
 		 * finally, insert request into driver dispatch list