blk-cgroup: don't update io stat for root cgroup

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2167245

commit 0416f3b
Author: Ming Lei <ming.lei@redhat.com>
Date:   Thu Feb 2 10:18:04 2023 +0800

    blk-cgroup: don't update io stat for root cgroup

    We source root cgroup stats from the system-wide stats, see blkcg_print_stat
    and blkcg_rstat_flush, so don't update io state for root cgroup.

    Fixes blkg leak issue introduced in commit 3b8cc62 ("blk-cgroup: Optimize blkcg_rstat_flush()")
    which starts to grab blkg's reference when adding iostat_cpu into percpu
    blkcg list, but this state won't be consumed by blkcg_rstat_flush() where
    the blkg reference is dropped.

    Tested-by: Bart van Assche <bvanassche@acm.org>
    Reported-by: Bart van Assche <bvanassche@acm.org>
    Fixes: 3b8cc62 ("blk-cgroup: Optimize blkcg_rstat_flush()")
    Cc: Tejun Heo <tj@kernel.org>
    Cc: Waiman Long <longman@redhat.com>
    Signed-off-by: Ming Lei <ming.lei@redhat.com>
    Link: https://lore.kernel.org/r/20230202021804.278582-1-ming.lei@redhat.com
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

block/blk-cgroup.c

@@ -1996,6 +1996,10 @@ void blk_cgroup_bio_start(struct bio *bio)
 	struct blkg_iostat_set *bis;
 	unsigned long flags;
 
+	/* Root-level stats are sourced from system-wide IO stats */
+	if (!cgroup_parent(blkcg->css.cgroup))
+		return;
+
 	cpu = get_cpu();
 	bis = per_cpu_ptr(bio->bi_blkg->iostat_cpu, cpu);
 	flags = u64_stats_update_begin_irqsave(&bis->sync);