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Thu, 27 Aug 2026 07:59:18 -0700 (PDT) Date: Thu, 27 Aug 2026 10:59:17 -0400 From: Johannes Weiner To: Jiayuan Chen Cc: linux-mm@kvack.org, stable@vger.kernel.org, syzbot+ced4d9a8cadb5ef3adae@syzkaller.appspotmail.com, Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , Yosry Ahmed , Chris Li , Jens Axboe , Jan Kara , Tejun Heo , cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/memcontrol: fix data-race on reading jiffies_64 Message-ID: <20260827145917.GB3004@cmpxchg.org> References: <20260827025457.116191-1-jiayuan.chen@linux.dev> Precedence: bulk X-Mailing-List: cgroups@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260827025457.116191-1-jiayuan.chen@linux.dev> On Thu, Aug 27, 2026 at 10:54:56AM +0800, Jiayuan Chen wrote: > KCSAN reported a data-race between tick_do_update_jiffies64() updating > jiffies_64 and mem_cgroup_flush_stats_ratelimited() reading it directly. > > Unlike jiffies, jiffies_64 is not volatile, so raw reads are plain > accesses and can even be torn on 32-bit. Use get_jiffies_64() instead, > and fix the same pattern in mem_cgroup_flush_foreign(). > > Cc: > Fixes: 508bed884767 ("mm: memcg: change flush_next_time to flush_last_time") > Fixes: 97b27821b485 ("writeback, memcg: Implement foreign dirty flushing") > Reported-by: syzbot+ced4d9a8cadb5ef3adae@syzkaller.appspotmail.com > Signed-off-by: Jiayuan Chen Acked-by: Johannes Weiner Minor nit: > @@ -785,7 +785,7 @@ void mem_cgroup_flush_stats(struct mem_cgroup *memcg) > void mem_cgroup_flush_stats_ratelimited(struct mem_cgroup *memcg) > { > /* Only flush if the periodic flusher is one full cycle late */ > - if (time_after64(jiffies_64, READ_ONCE(flush_last_time) + 2*FLUSH_TIME)) > + if (time_after64(get_jiffies_64(), READ_ONCE(flush_last_time) + 2 * FLUSH_TIME)) > mem_cgroup_flush_stats(memcg); time_is_after_jiffies64()