A data-race issue in memcg rstat occurs when two distinct code paths
access the same 4-byte region concurrently.  KCSAN detection triggers the
following BUG as a result.

	BUG: KCSAN: data-race in __count_memcg_events / mem_cgroup_css_rstat_flush

	write to 0xffffe8ffff98e300 of 4 bytes by task 5274 on cpu 17:
	mem_cgroup_css_rstat_flush (mm/memcontrol.c:5850)
	cgroup_rstat_flush_locked (kernel/cgroup/rstat.c:243 (discriminator 7))
	cgroup_rstat_flush (./include/linux/spinlock.h:401 kernel/cgroup/rstat.c:278)
	mem_cgroup_flush_stats.part.0 (mm/memcontrol.c:767)
	memory_numa_stat_show (mm/memcontrol.c:6911)
<snip>

	read to 0xffffe8ffff98e300 of 4 bytes by task 410848 on cpu 27:
	__count_memcg_events (mm/memcontrol.c:725 mm/memcontrol.c:962)
	count_memcg_event_mm.part.0 (./include/linux/memcontrol.h:1097 ./include/linux/memcontrol.h:1120)
	handle_mm_fault (mm/memory.c:5483 mm/memory.c:5622)
<snip>

	value changed: 0x00000029 -> 0x00000000

The race occurs because two code paths access the same "stats_updates"
location.  Although "stats_updates" is a per-CPU variable, it is remotely
accessed by another CPU at
cgroup_rstat_flush_locked()->mem_cgroup_css_rstat_flush(), leading to the
data race mentioned.

Considering that memcg_rstat_updated() is in the hot code path, adding a
lock to protect it may not be desirable, especially since this variable
pertains solely to statistics.

Therefore, annotating accesses to stats_updates with READ/WRITE_ONCE() can
prevent KCSAN splats and potential partial reads/writes.

Link: https://lkml.kernel.org/r/20240424125940.2410718-1-leitao@debian.org
Fixes: 9cee7e8e ("mm: memcg: optimize parent iteration in memcg_rstat_updated()")
Signed-off-by: Breno Leitao <leitao@debian.org>
Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Yosry Ahmed <yosryahmed@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Muchun Song <songmuchun@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>