blkio: Drop the reference to queue once the task changes cgroup

o If a task changes cgroup, drop reference to the cfqq associated with io
  context and set cfqq pointer stored in ioc to NULL so that upon next request
  arrival we will allocate a  new queue in new group.
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

block/cfq-iosched.c

@@ -2608,6 +2608,41 @@ static void cfq_init_cfqq(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 	cfqq->pid = pid;
 }
 
+#ifdef CONFIG_CFQ_GROUP_IOSCHED
+static void changed_cgroup(struct io_context *ioc, struct cfq_io_context *cic)
+{
+	struct cfq_queue *sync_cfqq = cic_to_cfqq(cic, 1);
+	struct cfq_data *cfqd = cic->key;
+	unsigned long flags;
+	struct request_queue *q;
+
+	if (unlikely(!cfqd))
+		return;
+
+	q = cfqd->queue;
+
+	spin_lock_irqsave(q->queue_lock, flags);
+
+	if (sync_cfqq) {
+		/*
+		 * Drop reference to sync queue. A new sync queue will be
+		 * assigned in new group upon arrival of a fresh request.
+		 */
+		cfq_log_cfqq(cfqd, sync_cfqq, "changed cgroup");
+		cic_set_cfqq(cic, NULL, 1);
+		cfq_put_queue(sync_cfqq);
+	}
+
+	spin_unlock_irqrestore(q->queue_lock, flags);
+}
+
+static void cfq_ioc_set_cgroup(struct io_context *ioc)
+{
+	call_for_each_cic(ioc, changed_cgroup);
+	ioc->cgroup_changed = 0;
+}
+#endif  /* CONFIG_CFQ_GROUP_IOSCHED */
+
 static struct cfq_queue *
 cfq_find_alloc_queue(struct cfq_data *cfqd, bool is_sync,
 		     struct io_context *ioc, gfp_t gfp_mask)
@@ -2840,6 +2875,10 @@ out:
 	if (unlikely(ioc->ioprio_changed))
 		cfq_ioc_set_ioprio(ioc);
 
+#ifdef CONFIG_CFQ_GROUP_IOSCHED
+	if (unlikely(ioc->cgroup_changed))
+		cfq_ioc_set_cgroup(ioc);
+#endif
 	return cic;
 err_free:
 	cfq_cic_free(cic);