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

[Michal Hocko <mhocko@suse.com>]

On Wed 24-04-24 05:59:39, Breno Leitao 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>

Acked-by: Michal Hocko <mhocko@suse.com>

It is worth mentioning that the race is harmless.

Thanks!

> ---
>  mm/memcontrol.c | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index fabce2b50c69..3c99457b36a1 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -715,6 +715,7 @@ static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
>  {
>  	struct memcg_vmstats_percpu *statc;
>  	int cpu = smp_processor_id();
> +	unsigned int stats_updates;
>  
>  	if (!val)
>  		return;
> @@ -722,8 +723,9 @@ static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
>  	cgroup_rstat_updated(memcg->css.cgroup, cpu);
>  	statc = this_cpu_ptr(memcg->vmstats_percpu);
>  	for (; statc; statc = statc->parent) {
> -		statc->stats_updates += abs(val);
> -		if (statc->stats_updates < MEMCG_CHARGE_BATCH)
> +		stats_updates = READ_ONCE(statc->stats_updates) + abs(val);
> +		WRITE_ONCE(statc->stats_updates, stats_updates);
> +		if (stats_updates < MEMCG_CHARGE_BATCH)
>  			continue;
>  
>  		/*
> @@ -731,9 +733,9 @@ static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
>  		 * redundant. Avoid the overhead of the atomic update.
>  		 */
>  		if (!memcg_vmstats_needs_flush(statc->vmstats))
> -			atomic64_add(statc->stats_updates,
> +			atomic64_add(stats_updates,
>  				     &statc->vmstats->stats_updates);
> -		statc->stats_updates = 0;
> +		WRITE_ONCE(statc->stats_updates, 0);
>  	}
>  }
>  
> @@ -5845,7 +5847,7 @@ static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu)
>  			}
>  		}
>  	}
> -	statc->stats_updates = 0;
> +	WRITE_ONCE(statc->stats_updates, 0);
>  	/* We are in a per-cpu loop here, only do the atomic write once */
>  	if (atomic64_read(&memcg->vmstats->stats_updates))
>  		atomic64_set(&memcg->vmstats->stats_updates, 0);
> -- 
> 2.43.0
> 

-- 
Michal Hocko
SUSE Labs