Received-SPF: pass (google.com: domain of linux-kernel+bounces-156944-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.48.161 as permitted sender) client-ip=147.75.48.161;
Date: Wed, 24 Apr 2024 09:19:18 -0400
From: Johannes Weiner <hannes@cmpxchg.org>
To: Breno Leitao <leitao@debian.org>
Cc: Michal Hocko <mhocko@kernel.org>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Shakeel Butt <shakeel.butt@linux.dev>,
	Muchun Song <muchun.song@linux.dev>,
	Andrew Morton <akpm@linux-foundation.org>, leit@meta.com,
	"open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)" <cgroups@vger.kernel.org>,
	"open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)" <linux-mm@kvack.org>,
	open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] memcg: Fix data-race KCSAN bug in rstats
Message-ID: <20240424131918.GC318022@cmpxchg.org>
References: <20240424125940.2410718-1-leitao@debian.org>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20240424125940.2410718-1-leitao@debian.org>

On Wed, Apr 24, 2024 at 05:59:39AM -0700, 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: Johannes Weiner <hannes@cmpxchg.org>