Re: [PATCH] arm: smp: Avoid false positive CPU hotplug Lockdep-RCU splat

[Russell King (Oracle)]

On Thu, Mar 07, 2024 at 09:45:36AM -0800, Paul E. McKenney wrote:
> > diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> > index 3431c0553f45..6875e2c5dd50 100644
> > --- a/arch/arm/kernel/smp.c
> > +++ b/arch/arm/kernel/smp.c
> > @@ -319,7 +319,14 @@ void __noreturn arch_cpu_idle_dead(void)
> >  {
> >  	unsigned int cpu = smp_processor_id();
> >  
> > +	/*
> > +	 * Briefly report CPU as online again to avoid false positive
> > +	 * Lockdep-RCU splat when check_and_switch_context() acquires ASID
> > +	 * spinlock.
> > +	 */
> > +	rcutree_report_cpu_starting(cpu);
> >  	idle_task_exit();
> > +	rcutree_report_cpu_dead();
> >  
> >  	local_irq_disable();
> 
> Both rcutree_report_cpu_starting() and rcutree_report_cpu_dead() complain
> bitterly via lockdep if interrupts are enabled.  And the call sites have
> interrupts disabled.  So I don't understand what this local_irq_disable()
> is needed for.

I think that's a question for this commit:

commit e78a7614f3876ac649b3df608789cb6ef74d0480
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date:   Wed Jun 5 07:46:43 2019 -0700

Before this commit, arch_cpu_idle_dead() was called with IRQs enabled.
This commit moved the local_irq_disable() before calling
arch_cpu_idle_dead() but it seems no one looked at the various arch
implementations to clean those up. Quite how arch people are supposed
to spot this and clean up after such a commit, I'm not sure.

The local_irq_disable() that you're asking about has been there ever
since the inception of SMP on 32-bit ARM in this commit:

commit a054a811597a17ffbe92bc4db04a4dc2f1b1ea55
Author: Russell King <rmk@xxxxxxxxxxxxxxxxxxxxxxx>
Date:   Wed Nov 2 22:24:33 2005 +0000

Where cpu_die() was later renamed to arch_cpu_idle_dead(). So it's
purely a case of a change being made to core code and arch code not
receiving any fixups for it.

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