On Sat, Oct 17, 2026 at 12:16:48PM +0100, Bradley Morgan wrote:
> On 17 October 2026 00:41:00 BST, Zqiang <[email protected]> wrote:
> >>
> >> On 15 October 2026 10:46:19 BST, Zqiang <[email protected]> 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.
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 <[email protected]>
> >> > ---
> >> > 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
