cfq: Don't allow queue merges for queues that have no process references

[mirror_ubuntu-bionic-kernel.git] / block / cfq-iosched.c

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 5ff4f4850e717ddb319423e9678e0e44cd7f265c..153f6277e5c8129e762cfe8ce143da4927d86934 100644 (file)
--- 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);