On 17 October 2026 14:37:24 BST, Zqiang <[email protected]> wrote: >> >> On 17 October 2026 13:20:50 BST, Bradley Morgan <[email protected]> >> wrote: >> >> > >> > On 17 October 2026 13:14:29 BST, Zqiang <[email protected]> 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# mailto:root@p10-linux-brads:~/linux# >mailto: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(); >> > } >> > >> > > >> > > RCU Tasks Rude use preempt_disable/enable as the read critical >section, >> > > you use smp_call_function_single() to send ipi to each online CPUs, >> > > it's not forces a context sched switch, and not wait the >> > > preempt_disable/enable >> > > read critical section to exit, this is the wrong mechanism. ;-( >> > > >> > > Obviously, we only need to not apply alloc_percpu dynamic >allocation: >> > > >> > > diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h >> > > index fcac7361ec51..49b629b0010b 100644 >> > > --- a/kernel/rcu/tasks.h >> > > +++ b/kernel/rcu/tasks.h >> > > @@ -1357,11 +1357,25 @@ static void rcu_tasks_be_rude(struct >work_struct >> > > >> > *work) >> > >> > > >> > > { >> > > } >> > > >> > > +static DEFINE_PER_CPU(struct work_struct, tasks_rude_work); >> > > + >> > > // Wait for one rude RCU-tasks grace period. >> > > static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp) >> > > { >> > > + int cpu; >> > > + >> > > + guard(cpus_read_lock)(); >> > > rtp->n_ipis += cpumask_weight(cpu_online_mask); >> > > - schedule_on_each_cpu(rcu_tasks_be_rude); >> > > + >> > > + for_each_online_cpu(cpu) { >> > > + struct work_struct *work = &per_cpu(tasks_rude_work, >> > > >> > cpu); >> > >> > > >> > > + >> > > + INIT_WORK(work, rcu_tasks_be_rude); >> > > + schedule_work_on(cpu, work); >> > > + } >> > > + >> > > + for_each_online_cpu(cpu) >> > > + flush_work(&per_cpu(tasks_rude_work, cpu)); >> > > } >> > > >> > > static void call_rcu_tasks_rude(struct rcu_head *rhp, >rcu_callback_t >> > > func); >> > > >> > Wow! >> > >> > Reviewed-by: Bradley Morgan <[email protected]> >> > >> Gimme a sec I'll submit the patch.. > >Actually, I don't mind you send this fix directly, due to you use an >incorrect IPI >mechanism to implement it, I would like to know wether you truly >understand the tasks-rude >and how schedule_on_each_cpu() ensures memory order.
Hi, I do understand how it all works. The reasons why you would suspect I wouldn't 1: I used a ai slop tool to suggest *my* fix, and I tested it on rcutorture 2: I'm sometimes a bit dumb, it happens :( You said you "don't mind", does that mean you DONT want your fix submitted (but I already submitted it ;c), or that you are okay with your fix being sent. > > >> >> > >> > > >> > > Thanks >> > > Zqiang >> > > >> > >> > static void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t >> > >> > > >> > > func); >> > > >> > root@p10-linux-brads:~/linux# mailto:root@p10-linux-brads:~/linux# >mailto: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 >> > >> > --- Thanks! >> > "I'm not a very positive person" - Linus torvalds >> > >> --- Thanks! >> "I'm not a very positive person" - Linus torvalds >> --- Thanks! https://lore.kernel.org/all/[email protected]/
