blkcg: add blkio_policy[] array and allow one policy per policy ID

Block cgroup policies are maintained in a linked list and,
theoretically, multiple policies sharing the same policy ID are
allowed.

This patch temporarily restricts one policy per plid and adds
blkio_policy[] array which indexes registered policy types by plid.
Both the restriction and blkio_policy[] array are transitional and
will be removed once API cleanup is complete.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -29,6 +29,8 @@ static LIST_HEAD(blkio_list);
 struct blkio_cgroup blkio_root_cgroup = { .weight = 2*BLKIO_WEIGHT_DEFAULT };
 EXPORT_SYMBOL_GPL(blkio_root_cgroup);
 
+static struct blkio_policy_type *blkio_policy[BLKIO_NR_POLICIES];
+
 static struct cgroup_subsys_state *blkiocg_create(struct cgroup_subsys *,
 						  struct cgroup *);
 static int blkiocg_can_attach(struct cgroup_subsys *, struct cgroup *,
@@ -1694,15 +1696,23 @@ static void blkiocg_attach(struct cgroup_subsys *ss, struct cgroup *cgrp,
 void blkio_policy_register(struct blkio_policy_type *blkiop)
 {
 	spin_lock(&blkio_list_lock);
+
+	BUG_ON(blkio_policy[blkiop->plid]);
+	blkio_policy[blkiop->plid] = blkiop;
 	list_add_tail(&blkiop->list, &blkio_list);
+
 	spin_unlock(&blkio_list_lock);
 }
 EXPORT_SYMBOL_GPL(blkio_policy_register);
 
 void blkio_policy_unregister(struct blkio_policy_type *blkiop)
 {
 	spin_lock(&blkio_list_lock);
+
+	BUG_ON(blkio_policy[blkiop->plid] != blkiop);
+	blkio_policy[blkiop->plid] = NULL;
 	list_del_init(&blkiop->list);
+
 	spin_unlock(&blkio_list_lock);
 }
 EXPORT_SYMBOL_GPL(blkio_policy_unregister);

block/blk-cgroup.h

@@ -19,6 +19,8 @@
 enum blkio_policy_id {
 	BLKIO_POLICY_PROP = 0,		/* Proportional Bandwidth division */
 	BLKIO_POLICY_THROTL,		/* Throttling */
+
+	BLKIO_NR_POLICIES,
 };
 
 /* Max limits for throttle policy */