Message-ID: <066e8121-c6c5-48ac-b35a-e6430d986dff@nvidia.com>
Date: Tue, 3 Jun 2025 15:03:30 -0400
From: Joel Fernandes <joelagnelf@...dia.com>
To: Xiongfeng Wang <wangxiongfeng2@...wei.com>
Cc: Joel Fernandes <joel@...lfernandes.org>, ankur.a.arora@...cle.com,
 Frederic Weisbecker <frederic@...nel.org>,
 "Paul E . McKenney" <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
 neeraj.upadhyay@...nel.org, urezki@...il.com, rcu@...r.kernel.org,
 linux-kernel@...r.kernel.org, xiqi2@...wei.com,
 "Wangshaobo (bobo)" <bobo.shaobowang@...wei.com>
Subject: Re: [QUESTION] problems report: rcu_read_unlock_special() called in
 irq_exit() causes dead loop



On 6/3/2025 2:59 PM, Joel Fernandes wrote:
> On Fri, May 30, 2025 at 09:55:45AM +0800, Xiongfeng Wang wrote:
>> Hi Joel,
>>
>> On 2025/5/29 0:30, Joel Fernandes wrote:
>>> On Wed, May 21, 2025 at 5:43 AM Xiongfeng Wang
>>> <wangxiongfeng2@...wei.com> wrote:
>>>>
>>>> Hi RCU experts,
>>>>
>>>> When I ran syskaller in Linux 6.6 with CONFIG_PREEMPT_RCU enabled, I got
>>>> the following soft lockup. The Calltrace is too long. I put it in the end.
>>>> The issue can also be reproduced in the latest kernel.
>>>>
>>>> The issue is as follows. CPU3 is waiting for a spin_lock, which is got by CPU1.
>>>> But CPU1 stuck in the following dead loop.
>>>>
>>>> irq_exit()
>>>>   __irq_exit_rcu()
>>>>     /* in_hardirq() returns false after this */
>>>>     preempt_count_sub(HARDIRQ_OFFSET)
>>>>     tick_irq_exit()
>>>>       tick_nohz_irq_exit()
>>>>             tick_nohz_stop_sched_tick()
>>>>               trace_tick_stop()  /* a bpf prog is hooked on this trace point */
>>>>                    __bpf_trace_tick_stop()
>>>>                       bpf_trace_run2()
>>>>                             rcu_read_unlock_special()
>>>>                               /* will send a IPI to itself */
>>>>                               irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
>>>>
>>>> /* after interrupt is enabled again, the irq_work is called */
>>>> asm_sysvec_irq_work()
>>>>   sysvec_irq_work()
>>>> irq_exit() /* after handled the irq_work, we again enter into irq_exit() */
>>>>   __irq_exit_rcu()
>>>>     ...skip...
>>>>            /* we queue a irq_work again, and enter a dead loop */
>>>>            irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
>>>
>>> This seems legitimate, Boqun and I were just talking about it. He may
>>> share more thoughts but here are a few:
>>>
>>> Maybe we can delay subsequent clearing of the flag in
>>> rcu_preempt_deferred_qs_handler() using a timer and an exponential
>>> back-off? That way we are not sending too many self-IPIs.
>>>
>>> And reset the process at the end of a grace period.
>>>
>>> Or just don't send subsequent self-IPIs if we just sent one for the
>>> rdp. Chances are, if we did not get the scheduler's attention during
>>> the first one, we may not in subsequent ones I think. Plus we do send
>>> other IPIs already if the grace period was over extended (from the FQS
>>> loop), maybe we can tweak that?
>>
>> Thanks a lot for your reply. I think it's hard for me to fix this issue as
>> above without introducing new bugs. I barely understand the RCU code. But I'm
>> very glad to help test if you have any code modifiction need to. I have
>> the VM and the syskaller benchmark which can reproduce the problem.
> 
> Sure, I understand. This is already incredibly valuable so thank you again.
> Will request for your testing help soon. I also have a test module now which
> can sort-off reproduce this. Keep you posted!
Oh sorry I meant to ask - could you provide the full kernel log and also is
there a standalone reproducer syzcaller binary one can run to reproduce it in a VM?

Thanks a lot.

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