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

[Zqiang]

> 
> On 15 October 2026 10:46:19 BST, Zqiang <qiang.zhang@xxxxxxxxx> 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.

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@xxxxxxxxx>
> > ---
> >  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
>