blkcg: add blkcg->id

Add 64bit unique id to blkcg. This will be used by policies which
want blkcg identity test to tell whether the associated blkcg has
changed.

Signed-off-by: Tejun Heo <[e-mail]>
Cc: Vivek Goyal <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

block/blk-cgroup.c

···
 #include <linux/slab.h>
 #include <linux/genhd.h>
 #include <linux/delay.h>
+#include <linux/atomic.h>
 #include "blk-cgroup.h"
 #include "blk.h"
 
···
 static struct cgroup_subsys_state *
 blkiocg_create(struct cgroup_subsys *subsys, struct cgroup *cgroup)
 {
+	static atomic64_t id_seq = ATOMIC64_INIT(0);
 	struct blkio_cgroup *blkcg;
 	struct cgroup *parent = cgroup->parent;
 
···
 		return ERR_PTR(-ENOMEM);
 
 	blkcg->weight = BLKIO_WEIGHT_DEFAULT;
+	blkcg->id = atomic64_inc_return(&id_seq); /* root is 0, start from 1 */
 done:
 	spin_lock_init(&blkcg->lock);
 	INIT_HLIST_HEAD(&blkcg->blkg_list);

block/blk-cgroup.h

···
 	unsigned int weight;
 	spinlock_t lock;
 	struct hlist_head blkg_list;
+
+	/* for policies to test whether associated blkcg has changed */
+	uint64_t id;
 };
 
 struct blkio_group_stats {