Re: [PATCH] memcg: Fix data-race KCSAN bug in rstats

[Yosry Ahmed <yosryahmed@google.com>]

On Wed, Apr 24, 2024 at 6:00 AM Breno Leitao <leitao@debian.org> wrote:
>
> 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.
>
> Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
> Signed-off-by: Breno Leitao <leitao@debian.org>

Reviewed-by: Yosry Ahmed <yosryahmed@google.com>

, and or posterity:
Fixes: 9cee7e8ef3e3 ("mm: memcg: optimize parent iteration in
memcg_rstat_updated()")