Re: [PATCH 09/19] rcu/context-tracking: Remove rcu_irq_enter/exit()

[Frederic Weisbecker]

On Sat, Mar 05, 2022 at 03:16:03PM +0100, Peter Zijlstra wrote:
> On Wed, Mar 02, 2022 at 04:48:00PM +0100, Frederic Weisbecker wrote:
> >  void ct_irq_enter_irqson(void)
> >  {
> > -	rcu_irq_enter_irqson();
> > +	unsigned long flags;
> > +
> > +	local_irq_save(flags);
> > +	ct_irq_enter();
> > +	local_irq_restore(flags);
> >  }
> >  
> >  void ct_irq_exit_irqson(void)
> >  {
> > -	rcu_irq_exit_irqson();
> > +	unsigned long flags;
> > +
> > +	local_irq_save(flags);
> > +	ct_irq_exit();
> > +	local_irq_restore(flags);
> >  }
> 
> I know you're just copying code around, but this is broken per
> construction :/
> 
> On the irq_enter site, local_irq_save() will hit a tracepoint, which
> requires RCU, which will only be made available by the ct_irq_enter().
> Same in reverse for the exit case.

Ouch. And playing a game similar to that in default_idle_call() is going to
be trickier because rcu_irq_enter() may or may not exit an RCU-off mode. But it's
feasible. Again I'll fix it ahead.

Thanks!