linux-kernel - Re: [syzbot] [kernel?] WARNING: suspicious RCU usage in __do

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Date: Tue, 23 Apr 2024 10:20:49 +0800
From: Z qiang <qiang.zhang1211@...il.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: paulmck@...nel.org, 
	syzbot <syzbot+dce04ed6d1438ad69656@...kaller.appspotmail.com>, 
	linux-kernel@...r.kernel.org, peterz@...radead.org, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [kernel?] WARNING: suspicious RCU usage in __do_softirq

>
> On Fri, Apr 19 2024 at 13:50, Z qiang wrote:
> >> On Thu, Apr 18, 2024 at 05:49:38PM +0800, Z qiang wrote:
> >  static __init int spawn_ksoftirqd(void)
> >  {
> > +       int cpu;
> > +
> > +       for_each_possible_cpu(cpu)
> > +               per_cpu(ksoftirqd_work, cpu) = false;
>
> First of all that initialization is pointless, but why making all of
> this complex as hell?
>
> Thanks,
>
>         tglx
> ---
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index b315b21fb28c..e991d735be0d 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -508,7 +508,7 @@ static inline bool lockdep_softirq_start(void) { return false; }
>  static inline void lockdep_softirq_end(bool in_hardirq) { }
>  #endif
>
> -asmlinkage __visible void __softirq_entry __do_softirq(void)
> +static void handle_softirqs(bool kirqd)
>  {
>         unsigned long end = jiffies + MAX_SOFTIRQ_TIME;
>         unsigned long old_flags = current->flags;
> @@ -563,8 +563,7 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
>                 pending >>= softirq_bit;
>         }
>
> -       if (!IS_ENABLED(CONFIG_PREEMPT_RT) &&
> -           __this_cpu_read(ksoftirqd) == current)
> +       if (!IS_ENABLED(CONFIG_PREEMPT_RT) && kirqd)
>                 rcu_softirq_qs();
>
>         local_irq_disable();
> @@ -584,6 +583,11 @@ asmlinkage __visible void __softirq_entry __do_softirq(void)
>         current_restore_flags(old_flags, PF_MEMALLOC);
>  }
>
> +asmlinkage __visible void __softirq_entry __do_softirq(void)
> +{
> +       handle_softirqs(false);
> +}
> +
>  /**
>   * irq_enter_rcu - Enter an interrupt context with RCU watching
>   */
> @@ -921,7 +925,7 @@ static void run_ksoftirqd(unsigned int cpu)
>                  * We can safely run softirq on inline stack, as we are not deep
>                  * in the task stack here.
>                  */
> -               __do_softirq();
> +               handle_softirqs(true);

Thanks, this is good for me :),
Paul, what do you think?

Thanks
Zqiang

>                 ksoftirqd_run_end();
>                 cond_resched();
>                 return;