netdev - Re: [PATCH net] net_sched: gen

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Message-ID: <CANn89iJgfe4ONBJ4emeWRAk1W_pO5HTMKcO6dHLt9zO-kZhbTw@mail.gmail.com>
Date: Tue, 26 Aug 2025 11:06:47 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	syzbot+72db9ee39db57c3fecc5@...kaller.appspotmail.com, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH net] net_sched: gen_estimator:

On Tue, Aug 26, 2025 at 11:05 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> syzbot reported a WARNING in est_timer() [1]
>
> Problem here is that with CONFIG_PREEMPT_RT=y, timer callbacks
> can be preempted.
>
> Adopt preempt_disable_nested()/preempt_enable_nested() to fix this.
>

Patch title is messed up.

It should read :

net_sched: gen_estimator: fix est_timer() vs CONFIG_PREEMPT_RT=y

> [1]
>  WARNING: CPU: 0 PID: 16 at ./include/linux/seqlock.h:221 __seqprop_assert include/linux/seqlock.h:221 [inline]
>  WARNING: CPU: 0 PID: 16 at ./include/linux/seqlock.h:221 est_timer+0x6dc/0x9f0 net/core/gen_estimator.c:93
> Modules linked in:
> CPU: 0 UID: 0 PID: 16 Comm: ktimers/0 Not tainted syzkaller #0 PREEMPT_{RT,(full)}
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
>  RIP: 0010:__seqprop_assert include/linux/seqlock.h:221 [inline]
>  RIP: 0010:est_timer+0x6dc/0x9f0 net/core/gen_estimator.c:93
> Call Trace:
>  <TASK>
>   call_timer_fn+0x17e/0x5f0 kernel/time/timer.c:1747
>   expire_timers kernel/time/timer.c:1798 [inline]
>   __run_timers kernel/time/timer.c:2372 [inline]
>   __run_timer_base+0x648/0x970 kernel/time/timer.c:2384
>   run_timer_base kernel/time/timer.c:2393 [inline]
>   run_timer_softirq+0xb7/0x180 kernel/time/timer.c:2403
>   handle_softirqs+0x22c/0x710 kernel/softirq.c:579
>   __do_softirq kernel/softirq.c:613 [inline]
>   run_ktimerd+0xcf/0x190 kernel/softirq.c:1043
>   smpboot_thread_fn+0x53f/0xa60 kernel/smpboot.c:160
>   kthread+0x70e/0x8a0 kernel/kthread.c:463
>   ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
>   ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>  </TASK>
>
> Fixes: d2d6422f8bd1 ("x86: Allow to enable PREEMPT_RT.")
> Reported-by: syzbot+72db9ee39db57c3fecc5@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/netdev/68adf6fa.a70a0220.3cafd4.0000.GAE@google.com/T/#u
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  net/core/gen_estimator.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/net/core/gen_estimator.c b/net/core/gen_estimator.c
> index 7d426a8e29f30b67655cf706060bfc6c478f3123..f112156db587ba54a995a68ead5a35fe0cb16230 100644
> --- a/net/core/gen_estimator.c
> +++ b/net/core/gen_estimator.c
> @@ -90,10 +90,12 @@ static void est_timer(struct timer_list *t)
>         rate = (b_packets - est->last_packets) << (10 - est->intvl_log);
>         rate = (rate >> est->ewma_log) - (est->avpps >> est->ewma_log);
>
> +       preempt_disable_nested();
>         write_seqcount_begin(&est->seq);
>         est->avbps += brate;
>         est->avpps += rate;
>         write_seqcount_end(&est->seq);
> +       preempt_enable_nested();
>
>         est->last_bytes = b_bytes;
>         est->last_packets = b_packets;
> --
> 2.51.0.261.g7ce5a0a67e-goog
>