Re: [PATCH] rcu-tasks: Warn on too-short RCU Tasks Rude grace period

[Bradley Morgan <brads@mainlining.org>]

On 17 October 2026 21:04:37 BST, "Paul E. McKenney" <paulmck@kernel.org>
wrote:
>On Sat, Oct 17, 2026 at 06:49:41PM +0100, Bradley Morgan wrote:
>> On 17 October 2026 18:46:25 BST, "Paul E. McKenney" <paulmck@kernel.org>
>> wrote:
>> >On Sat, Oct 17, 2026 at 12:16:48PM +0100, Bradley Morgan wrote:
>> >> On 17 October 2026 00:41:00 BST, Zqiang <qiang.zhang@linux.dev>
>wrote:
>> >> >> 
>> >> >> On 15 October 2026 10:46:19 BST, Zqiang <qiang.zhang@linux.dev>
>> >wrote:
>> >> >> 
>> >> >> > 
>> >> >> > The RCU Tasks Rude grace period is implemented by invoking the
>> >> >> > schedule_on_each_cpu() with an empty work function, which forces
>> >> >> > a context switch on every online CPU and guarantees that all
>> >> >> > rude-tasks read-side critical sections have completed. However,
>> >> >> > schedule_on_each_cpu() can fail (via alloc_percpu() returning
>> >> >> > -ENOMEM) and returns immediately, in which case those context
>> >> >> > switches never happen, too-short grace period happen.
>> >> >> > 
>> >> >> > This commit therefore wraps the schedule_on_each_cpu() call in
>> >> >> > WARN_ONCE() so that this failure is reported as a "Too-short
>> >> >> > grace period" warning.
>> >> >> > 
>> >> >> NAK, this seems like a edge case, and potentially panicking the
>> >system
>> >> >> with panic on oops doesn't seem right to me, because we would
>reserve
>> >> >that
>> >> >> for when something definitely goes really wrong and we need to
>notify
>> >> >> the user.
>> >> >> 
>> >> >> I don't believe this patch should go in for this reason, it's a
>heavy
>> >> >edge
>> >> >> case, that doesn't *have* nessesarily need to be accounted for.
>> >> >
>> >> >Indeed, this is a rare event, so I use WARN_ONCE() to mark it, the
>> >> >WARN_ONCE()
>> >> >not panicking system unless you set warn_on_oops true, this can help
>us
>> >> >troubleshoot
>> >> >UAF issues caused by a too short tasks-rude grace period.
>> >> >
>> >> >or maybe we can use DEFINE_PER_CPU(struct work_struct, rude_work) to
>> >> >replace
>> >> >alloc_percpu(struct work_struct) queue to all online CPUs.
>> >> >
>> >> >Anyway, Thank you for your reply. I would also like to hear the
>> >opinions
>> >> >of others.
>> >> 
>> >> Fair enough, but me and Paul done some discussions:
>> >> 
>> >> Here is the TL;DR
>> >> 
>> >> My ^W^Wai slop^W^W tool helped me with this: if the too short grace
>> >> happens,
>> >> it falls back.
>> >> 
>> >> 
>> >> root@p10-linux-brads:~/linux# git diff kernel/rcu
>> >> diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
>> >> index 627295396cd9..65bb7d4ed632 100644
>> >> --- a/kernel/rcu/tasks.h
>> >> +++ b/kernel/rcu/tasks.h
>> >> @@ -1344,15 +1344,23 @@ void exit_tasks_rcu_finish(void) { }
>> >>  // Ordering is provided by the scheduler's context-switch code.
>> >> 
>> >>  // Empty function to allow workqueues to force a context switch.
>> >> -static void rcu_tasks_be_rude(struct work_struct *work)
>> >> +static void __maybe_unused rcu_tasks_be_rude(struct work_struct
>*work)
>> >>  {
>> >>  }
>> >> 
>> >>  // Wait for one rude RCU-tasks grace period.
>> >> +static void rcu_tasks_rude_ipi(void *info)
>> >> +{
>> >> +}
>> >> +
>> >>  static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
>> >>  {
>> >> +       int cpu;
>> >>         rtp->n_ipis += cpumask_weight(cpu_online_mask);
>> >> -       schedule_on_each_cpu(rcu_tasks_be_rude);
>> >> +       cpus_read_lock();
>> >> +       for_each_online_cpu(cpu)
>> >> +               smp_call_function_single(cpu, rcu_tasks_rude_ipi,
>NULL,
>> >1);
>> >> +       cpus_read_unlock();
>> >>  }
>> >> 
>> >>  static void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t
>> >func);
>> >> root@p10-linux-brads:~/linux#
>> >> 
>> >> On POWERPC, this passes 12 hours of rcutorture on the RUDE01 config.
>> >
>> >That is surprising, given that when I run your patch shown above on my
>> >x86 laptop, rcutorture reports a too-short grace period within 20
>seconds.
>> 
>> Odd! I definitely ran it for 12 hours, on PowerPC, so this is certainly
>odd
>
>Those who care are free run it themselves on the systems of their choice.


root@p10-linux-brads:~/linux/tools/testing/selftests/rcutorture/res/2026.10.15-19.45.43/RUDE01#
uname -a && uname
Linux p10-linux-brads 7.0.0-34-generic #34-Ubuntu SMP PREEMPT_DYNAMIC Wed
Sep  2 16:05:10 UTC 2026 ppc64le GNU/Linux
Linux
root@p10-linux-brads:~/linux/tools/testing/selftests/rcutorture/res/2026.10.15-19.45.43/RUDE01#
tail -n 20 console.log
[43269.029077][   T73] tasks-rude-torture: rcu_torture_read_exit: Start of
episode
[43269.033954][   T73] tasks-rude-torture: rcu_torture_read_exit: End of
episode
[43282.341070][   T73] tasks-rude-torture: rcu_torture_read_exit: Start of
episode
[43282.345268][   T73] tasks-rude-torture: rcu_torture_read_exit: End of
episode
[43295.653087][   T73] tasks-rude-torture: rcu_torture_read_exit: Start of
episode
[43295.657260][   T73] tasks-rude-torture: rcu_torture_read_exit: End of
episode
[43308.965081][   T73] tasks-rude-torture: rcu_torture_read_exit: Start of
episode
[43308.969151][   T73] tasks-rude-torture: rcu_torture_read_exit: End of
episode
[43322.277071][   T73] tasks-rude-torture: rcu_torture_read_exit: Start of
episode
[43322.281345][   T73] tasks-rude-torture: rcu_torture_read_exit: End of
episode
[43335.589160][   T73] tasks-rude-torture: rcu_torture_read_exit: Start of
episode
[43335.593159][   T73] tasks-rude-torture: rcu_torture_read_exit: End of
episode
[43348.901077][   T73] tasks-rude-torture: rcu_torture_read_exit: Start of
episode
[43348.905165][   T73] tasks-rude-torture: rcu_torture_read_exit: End of
episode
[43362.213069][   T73] tasks-rude-torture: rcu_torture_read_exit: Start of
episode
[43362.217298][   T73] tasks-rude-torture: rcu_torture_read_exit: End of
episode
[43375.525085][   T73] tasks-rude-torture: rcu_torture_read_exit: Start of
episode
[43375.529207][   T73] tasks-rude-torture: rcu_torture_read_exit: End of
episode
[43388.837074][   T73] tasks-rude-torture: rcu_torture_read_exit: Start of
episode
[43388.841160][   T73] tasks-rude-torture: rcu_torture_read_exit: End of
episode
root@p10-linux-brads:~/linux/tools/testing/selftests/rcutorture/res/2026.10.15-19.45.43/RUDE01#

I certainly ran it :(


>
>							Thanx, Paul
>
>> >> If you plan on submitting this into a patch, please add my
>authorship,
>> >or
>> >> my c-d-b. 
>> >> 
>> >> Feel free to rcutorture test it ;d
>> >> 
>> >> BTW: have you reproduced this on your own system?
>> >> 
>> >> >
>> >> >Thanks
>> >> >Zqiang
>> >> >
>> >> >
>> >> >> 
>> >> >> Zqiang, have you reproduced this OOM on your own system, if so,
>could
>> >> >you
>> >> >> tell me more details, you may turn my frown into a smiley face ;)
>> >> >> 
>> >> >> > 
>> >> >> > Signed-off-by: Zqiang <qiang.zhang@linux.dev>
>> >> >> > ---
>> >> >> >  kernel/rcu/tasks.h | 3 ++-
>> >> >> >  1 file changed, 2 insertions(+), 1 deletion(-)
>> >> >> > 
>> >> >> > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
>> >> >> > index fcac7361ec51..91c25717402e 100644
>> >> >> > --- a/kernel/rcu/tasks.h
>> >> >> > +++ b/kernel/rcu/tasks.h
>> >> >> > @@ -1361,7 +1361,8 @@ static void rcu_tasks_be_rude(struct
>> >work_struct
>> >> >*work)
>> >> >> >  static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
>> >> >> >  {
>> >> >> >  rtp->n_ipis += cpumask_weight(cpu_online_mask);
>> >> >> > - schedule_on_each_cpu(rcu_tasks_be_rude);
>> >> >> > + WARN_ONCE(schedule_on_each_cpu(rcu_tasks_be_rude),
>> >> >> > + "%s: Too-short grace period, OOM is now expected
>behavior!!!\n",
>> >> >rtp->name);
>> >> >> > 
>> >> >> Good message, makes sense. But I don't think this is right, see
>> >above.
>> >> >> 
>> >> >> > 
>> >> >> > }
>> >> >> >  
>> >> >> >  static void call_rcu_tasks_rude(struct rcu_head *rhp,
>> >rcu_callback_t
>> >> >> >  func);
>> >> >> > 
>> >> >> --- Thanks!
>> >> >> "I'm not a very positive person" - Linus torvalds
>> >> >>
>> >> 
>> >> --- Thanks!
>> >> "I'm not a very positive person" - Linus torvalds
>> 
>> --- Thanks!
>> "I'm not a very positive person" - Linus torvalds

--- Thanks!
"I'm not a very positive person" - Linus torvalds