cfq-iosched: Don't update group weights when on service tree

Version 3 is updated to apply to for-2.6.39/core.

For version 2, I took Vivek's advice and made sure we update the group
weight from cfq_group_service_tree_add().

If a weight was updated while a group is on the service tree, the
calculation for the total weight of the service tree can be adjusted
improperly, which either leads to bad service tree weights, or
potentially crashes (if total_weight becomes 0).

This patch defers updates to the weight until a group is off the service
tree.

Signed-off-by: Justin TerAvest <[e-mail]>
Acked-by: Vivek Goyal <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

block/cfq-iosched.c

···
 	/* group service_tree key */
 	u64 vdisktime;
 	unsigned int weight;
+	unsigned int new_weight;
+	bool needs_update;
 
 	/* number of cfqq currently on this group */
 	int nr_cfqq;
···
 }
 
 static void
-cfq_group_service_tree_add(struct cfq_data *cfqd, struct cfq_group *cfqg)
+cfq_update_group_weight(struct cfq_group *cfqg)
+{
+	BUG_ON(!RB_EMPTY_NODE(&cfqg->rb_node));
+	if (cfqg->needs_update) {
+		cfqg->weight = cfqg->new_weight;
+		cfqg->needs_update = false;
+	}
+}
+
+static void
+cfq_group_service_tree_add(struct cfq_rb_root *st, struct cfq_group *cfqg)
+{
+	BUG_ON(!RB_EMPTY_NODE(&cfqg->rb_node));
+
+	cfq_update_group_weight(cfqg);
+	__cfq_group_service_tree_add(st, cfqg);
+	st->total_weight += cfqg->weight;
+}
+
+static void
+cfq_group_notify_queue_add(struct cfq_data *cfqd, struct cfq_group *cfqg)
 {
 	struct cfq_rb_root *st = &cfqd->grp_service_tree;
 	struct cfq_group *__cfqg;
···
 		cfqg->vdisktime = __cfqg->vdisktime + CFQ_IDLE_DELAY;
 	} else
 		cfqg->vdisktime = st->min_vdisktime;
-
-	__cfq_group_service_tree_add(st, cfqg);
-	st->total_weight += cfqg->weight;
+	cfq_group_service_tree_add(st, cfqg);
 }
 
 static void
-cfq_group_service_tree_del(struct cfq_data *cfqd, struct cfq_group *cfqg)
+cfq_group_service_tree_del(struct cfq_rb_root *st, struct cfq_group *cfqg)
+{
+	st->total_weight -= cfqg->weight;
+	if (!RB_EMPTY_NODE(&cfqg->rb_node))
+		cfq_rb_erase(&cfqg->rb_node, st);
+}
+
+static void
+cfq_group_notify_queue_del(struct cfq_data *cfqd, struct cfq_group *cfqg)
 {
 	struct cfq_rb_root *st = &cfqd->grp_service_tree;
 
···
 		return;
 
 	cfq_log_cfqg(cfqd, cfqg, "del_from_rr group");
-	st->total_weight -= cfqg->weight;
-	if (!RB_EMPTY_NODE(&cfqg->rb_node))
-		cfq_rb_erase(&cfqg->rb_node, st);
+	cfq_group_service_tree_del(st, cfqg);
 	cfqg->saved_workload_slice = 0;
 	cfq_blkiocg_update_dequeue_stats(&cfqg->blkg, 1);
 }
···
 		charge = cfqq->allocated_slice;
 
 	/* Can't update vdisktime while group is on service tree */
-	cfq_rb_erase(&cfqg->rb_node, st);
+	cfq_group_service_tree_del(st, cfqg);
 	cfqg->vdisktime += cfq_scale_slice(charge, cfqg);
-	__cfq_group_service_tree_add(st, cfqg);
+	/* If a new weight was requested, update now, off tree */
+	cfq_group_service_tree_add(st, cfqg);
 
 	/* This group is being expired. Save the context */
 	if (time_after(cfqd->workload_expires, jiffies)) {
···
 void cfq_update_blkio_group_weight(void *key, struct blkio_group *blkg,
 					unsigned int weight)
 {
-	cfqg_of_blkg(blkg)->weight = weight;
+	struct cfq_group *cfqg = cfqg_of_blkg(blkg);
+	cfqg->new_weight = weight;
+	cfqg->needs_update = true;
 }
 
 static struct cfq_group *
···
 	service_tree->count++;
 	if ((add_front || !new_cfqq) && !group_changed)
 		return;
-	cfq_group_service_tree_add(cfqd, cfqq->cfqg);
+	cfq_group_notify_queue_add(cfqd, cfqq->cfqg);
 }
 
 static struct cfq_queue *
···
 		cfqq->p_root = NULL;
 	}
 
-	cfq_group_service_tree_del(cfqd, cfqq->cfqg);
+	cfq_group_notify_queue_del(cfqd, cfqq->cfqg);
 	BUG_ON(!cfqd->busy_queues);
 	cfqd->busy_queues--;
 	if (cfq_cfqq_sync(cfqq))