On 17 October 2026 18:46:25 BST, "Paul E. McKenney" <[email protected]> wrote: >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.
Odd! I definitely ran it for 12 hours, on PowerPC, so this is certainly odd > > 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 --- Thanks! "I'm not a very positive person" - Linus torvalds
