Re: [PATCH] x86/xen: remove unneeded preempt_disable() from xen_irq_enable()

[Juergen Gross]

On 21.09.21 10:11, Jan Beulich wrote:
> On 21.09.2021 09:58, Juergen Gross wrote:
> > On 21.09.21 09:53, Jan Beulich wrote:
> > > On 21.09.2021 09:02, Juergen Gross wrote:
> > > > --- a/arch/x86/xen/irq.c
> > > > +++ b/arch/x86/xen/irq.c
> > > > @@ -57,24 +57,20 @@ asmlinkage __visible void xen_irq_enable(void)
> > > >    {
> > > >    	struct vcpu_info *vcpu;
> > > >    
> > > > -	/*
> > > > -	 * We may be preempted as soon as vcpu->evtchn_upcall_mask is
> > > > -	 * cleared, so disable preemption to ensure we check for
> > > > -	 * events on the VCPU we are still running on.
> > > > -	 */
> > > > -	preempt_disable();
> > > > -
> > > >    	vcpu = this_cpu_read(xen_vcpu);
> > > >    	vcpu->evtchn_upcall_mask = 0;
> > > >    
> > > > -	/* Doesn't matter if we get preempted here, because any
> > > > -	   pending event will get dealt with anyway. */
> > > > +	/*
> > > > +	 * Now preemption could happen, but this is only possible if an event
> > > > +	 * was handled, so missing an event due to preemption is not
> > > > +	 * possible at all.
> > > > +	 * The worst possible case is to be preempted and then check events
> > > > +	 * pending on the old vcpu, but this is not problematic.
> > > > +	 */
> > >
> > > I agree this isn't problematic from a functional perspective, but ...
> > >
> > > >    	barrier(); /* unmask then check (avoid races) */
> > > >    	if (unlikely(vcpu->evtchn_upcall_pending))
> > > >    		xen_force_evtchn_callback();
> > >
> > > ... is a stray call here cheaper than ...
> > >
> > > > -
> > > > -	preempt_enable();
> > >
> > > ... the preempt_{dis,en}able() pair?
> >
> > The question is if a stray call in case of preemption (very unlikely)
> > is cheaper than the preempt_{dis|en}able() pair on each IRQ enabling.
> >
> > I'm quite sure removing the preempt_*() calls will be a net benefit.
>
> Well, yes, I agree. It would have been nice if the description pointed
> out the fact that preemption kicking in precisely here is very unlikely.
> But perhaps that's considered rather obvious ... The issue I'm having
> is with the prior comments: They indicated that preemption happening
> before the "pending" check would be okay, _despite_ the
> preempt_{dis,en}able() pair. One could view this as an indication that
> this pair was put there for another reason (e.g. to avoid the stray
> calls). But it may of course also be that the comment simply was stale.

The comment is older than the preempt_*() calls.

Those were added 8 years ago claiming they'd prevent lost events, but
at the same time at lease one other patch was added which really
prevented lost events, so adding the preempt_*() calls might just have
been a guess at that time.

> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Thanks,


Juergen

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