blkcg: remove blkio_group->stats_lock

With recent plug merge updates, all non-percpu stat updates happen
under queue_lock making stats_lock unnecessary to synchronize stat
updates.  The only synchronization necessary is stat reading, which
can be done using u64_stats_sync instead.

This patch removes blkio_group->stats_lock and adds
blkio_group_stats->syncp for reader synchronization.
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.h

@@ -119,6 +119,7 @@ struct blkio_cgroup {
 };
 
 struct blkio_group_stats {
+	struct u64_stats_sync syncp;
 	/* total disk time and nr sectors dispatched by this group */
 	uint64_t time;
 	uint64_t stat_arr[BLKIO_STAT_ARR_NR][BLKIO_STAT_TOTAL];
@@ -200,8 +201,6 @@ struct blkio_group {
 	/* reference count */
 	int refcnt;
 
-	/* Need to serialize the stats in the case of reset/update */
-	spinlock_t stats_lock;
 	struct blkg_policy_data *pd[BLKIO_NR_POLICIES];
 
 	/* List of blkg waiting for per cpu stats memory to be allocated */