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-rw-r--r--block/cfq-iosched.c13
1 files changed, 11 insertions, 2 deletions
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 5ff4f4850e7..153f6277e5c 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1986,6 +1986,15 @@ static void cfq_setup_merge(struct cfq_queue *cfqq, struct cfq_queue *new_cfqq)
int process_refs, new_process_refs;
struct cfq_queue *__cfqq;
+ /*
+ * If there are no process references on the new_cfqq, then it is
+ * unsafe to follow the ->new_cfqq chain as other cfqq's in the
+ * chain may have dropped their last reference (not just their
+ * last process reference).
+ */
+ if (!cfqq_process_refs(new_cfqq))
+ return;
+
/* Avoid a circular list and skip interim queue merges */
while ((__cfqq = new_cfqq->new_cfqq)) {
if (__cfqq == cfqq)
@@ -1994,17 +2003,17 @@ static void cfq_setup_merge(struct cfq_queue *cfqq, struct cfq_queue *new_cfqq)
}
process_refs = cfqq_process_refs(cfqq);
+ new_process_refs = cfqq_process_refs(new_cfqq);
/*
* If the process for the cfqq has gone away, there is no
* sense in merging the queues.
*/
- if (process_refs == 0)
+ if (process_refs == 0 || new_process_refs == 0)
return;
/*
* Merge in the direction of the lesser amount of work.
*/
- new_process_refs = cfqq_process_refs(new_cfqq);
if (new_process_refs >= process_refs) {
cfqq->new_cfqq = new_cfqq;
atomic_add(process_refs, &new_cfqq->ref);