Re: [PATCH] mm/memcontrol: fix data-race on reading jiffies_64

[Johannes Weiner]

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: <stable@xxxxxxxxxxxxxxx>
> Fixes: 508bed884767 ("mm: memcg: change flush_next_time to flush_last_time")
> Fixes: 97b27821b485 ("writeback, memcg: Implement foreign dirty flushing")
> Reported-by: syzbot+ced4d9a8cadb5ef3adae@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Jiayuan Chen <jiayuan.chen@xxxxxxxxx>

Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

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()