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

[Paul E. McKenney]

On Thu, Mar 07, 2024 at 06:54:38PM +0000, Russell King (Oracle) wrote:
> 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.

Telepathy?  ;-)

> 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.

Thank you for the info!

							Thanx, Paul