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

[Juergen Gross <jgross@suse.com>]

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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@suse.com>

Thanks,


Juergen


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