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Date:   Thu, 12 May 2022 13:06:25 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Liu Jian <liujian56@...wei.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Marco Elver <elver@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     David Miller <davem@...emloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] tcp: Add READ_ONCE() to read tcp_orphan_count

On Thu, May 12, 2022 at 3:32 AM Liu Jian <liujian56@...wei.com> wrote:
>
> The tcp_orphan_count per-CPU variable is read locklessly, so this commit
> add the READ_ONCE() to a load in order to avoid below KCSAN warnning:
>
> BUG: KCSAN: data-race in tcp_orphan_count_sum net/ipv4/tcp.c:2476 [inline]
> BUG: KCSAN: data-race in tcp_orphan_update+0x64/0x100 net/ipv4/tcp.c:2487
>
> race at unknown origin, with read to 0xffff9c63bbdac7a8 of 4 bytes by interrupt on cpu 2:
>  tcp_orphan_count_sum net/ipv4/tcp.c:2476 [inline]
>  tcp_orphan_update+0x64/0x100 net/ipv4/tcp.c:2487
>  call_timer_fn+0x33/0x210 kernel/time/timer.c:1414
>
> Fixes: 19757cebf0c5 ("tcp: switch orphan_count to bare per-cpu counters")
> Signed-off-by: Liu Jian <liujian56@...wei.com>
> ---
>  net/ipv4/tcp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index cf18fbcbf123..7245609f41e6 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -2718,7 +2718,7 @@ int tcp_orphan_count_sum(void)
>         int i, total = 0;
>
>         for_each_possible_cpu(i)
> -               total += per_cpu(tcp_orphan_count, i);
> +               total += READ_ONCE(per_cpu(tcp_orphan_count, i));

We might raise the discussion to lkml and/or KCSAN supporters.

Presumably, all per_cpu() uses in the kernel will have the same issue ?

By definition per-cpu data can be changed by other cpus.

So maybe per_cpu() should contain the annotation, instead of having to
annotate all users.


>
>         return max(total, 0);
>  }
> --
> 2.17.1
>

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