[PATCH v2 net] net_sched: gen_estimator: fix est_timer() vs CONFIG_PREEMPT_RT=y

[PATCH v2 net] net_sched: gen_estimator: fix est_timer() vs CONFIG_PREEMPT_RT=y

[Eric Dumazet]

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.

[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@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/68adf6fa.a70a0220.3cafd4.0000.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
v2: fixed patch title, CC net/sched maintainers.
 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

Re: [PATCH v2 net] net_sched: gen_estimator: fix est_timer() vs CONFIG_PREEMPT_RT=y

[Sebastian Andrzej Siewior]

On 2025-08-27 16:23:52 [+0000], Eric Dumazet 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.
…
> Fixes: d2d6422f8bd1 ("x86: Allow to enable PREEMPT_RT.")
> Reported-by: syzbot+72db9ee39db57c3fecc5@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/netdev/68adf6fa.a70a0220.3cafd4.0000.GAE@google.com/T/#u
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Sebastian

Re: [PATCH v2 net] net_sched: gen_estimator: fix est_timer() vs CONFIG_PREEMPT_RT=y

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Jakub Kicinski <kuba@kernel.org>:

On Wed, 27 Aug 2025 16:23:52 +0000 you 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.
> 
> [...]

Here is the summary with links:
  - [v2,net] net_sched: gen_estimator: fix est_timer() vs CONFIG_PREEMPT_RT=y
    https://git.kernel.org/netdev/net/c/9f74c0ea9b26

You are awesome, thank you!
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