Re: [PATCH v3] cgroup: llist: avoid memory tears for llist_node

[Tejun Heo <tj@kernel.org>]

On Fri, Jul 04, 2025 at 11:08:04AM -0700, Shakeel Butt wrote:
> Before the commit 36df6e3dbd7e ("cgroup: make css_rstat_updated nmi
> safe"), the struct llist_node is expected to be private to the one
> inserting the node to the lockless list or the one removing the node
> from the lockless list. After the mentioned commit, the llist_node in
> the rstat code is per-cpu shared between the stacked contexts i.e.
> process, softirq, hardirq & nmi. It is possible the compiler may tear
> the loads or stores of llist_node. Let's avoid that.
> 
> KCSAN reported the following race:
> 
>  Reported by Kernel Concurrency Sanitizer on:
>  CPU: 60 UID: 0 PID: 5425 ... 6.16.0-rc3-next-20250626 #1 NONE
>  Tainted: [E]=UNSIGNED_MODULE
>  Hardware name: ...
>  ==================================================================
>  ==================================================================
>  BUG: KCSAN: data-race in css_rstat_flush / css_rstat_updated
>  write to 0xffffe8fffe1c85f0 of 8 bytes by task 1061 on cpu 1:
>   css_rstat_flush+0x1b8/0xeb0
>   __mem_cgroup_flush_stats+0x184/0x190
>   flush_memcg_stats_dwork+0x22/0x50
>   process_one_work+0x335/0x630
>   worker_thread+0x5f1/0x8a0
>   kthread+0x197/0x340
>   ret_from_fork+0xd3/0x110
>   ret_from_fork_asm+0x11/0x20
>  read to 0xffffe8fffe1c85f0 of 8 bytes by task 3551 on cpu 15:
>   css_rstat_updated+0x81/0x180
>   mod_memcg_lruvec_state+0x113/0x2d0
>   __mod_lruvec_state+0x3d/0x50
>   lru_add+0x21e/0x3f0
>   folio_batch_move_lru+0x80/0x1b0
>   __folio_batch_add_and_move+0xd7/0x160
>   folio_add_lru_vma+0x42/0x50
>   do_anonymous_page+0x892/0xe90
>   __handle_mm_fault+0xfaa/0x1520
>   handle_mm_fault+0xdc/0x350
>   do_user_addr_fault+0x1dc/0x650
>   exc_page_fault+0x5c/0x110
>   asm_exc_page_fault+0x22/0x30
>  value changed: 0xffffe8fffe18e0d0 -> 0xffffe8fffe1c85f0
> 
> $ ./scripts/faddr2line vmlinux css_rstat_flush+0x1b8/0xeb0
> css_rstat_flush+0x1b8/0xeb0:
> init_llist_node at include/linux/llist.h:86
> (inlined by) llist_del_first_init at include/linux/llist.h:308
> (inlined by) css_process_update_tree at kernel/cgroup/rstat.c:148
> (inlined by) css_rstat_updated_list at kernel/cgroup/rstat.c:258
> (inlined by) css_rstat_flush at kernel/cgroup/rstat.c:389
> 
> $ ./scripts/faddr2line vmlinux css_rstat_updated+0x81/0x180
> css_rstat_updated+0x81/0x180:
> css_rstat_updated at kernel/cgroup/rstat.c:90 (discriminator 1)
> 
> These are expected race and a simple READ_ONCE/WRITE_ONCE resolves these
> reports. However let's add comments to explain the race and the need for
> memory barriers if stronger guarantees are needed.
> 
> More specifically the rstat updater and the flusher can race and cause a
> scenario where the stats updater skips adding the css to the lockless
> list but the flusher might not see those updates done by the skipped
> updater. This is benign race and the subsequent flusher will flush those
> stats and at the moment there aren't any rstat users which are not fine
> with this kind of race. However some future user might want more
> stricter guarantee, so let's add appropriate comments to ease the job of
> future users.
> 
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
> Fixes: 36df6e3dbd7e ("cgroup: make css_rstat_updated nmi safe")

Applied to cgroup/for-6.17. Sorry about the delay. I'm on a vacation and
ended up a lot more offline than I expected to be.

Thanks.

-- 
tejun