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.
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
