Re: [Qemu-devel] [RFC PATCH v4 13/23] cpus: only take BQL for sleeping threads

["Pavel Dovgalyuk" <dovgaluk@ispras.ru>]

> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> On 19/01/2018 13:36, Pavel Dovgalyuk wrote:
> >> From: Paolo Bonzini [mailto:pbonzini@redhat.com]
> >> On 19/01/2018 13:25, Pavel Dovgalyuk wrote:
> >>>>> It means, that I'll have to fix all the has_work function to avoid races,
> >>>>> because x86_cpu_has_work may have them?
> >>>> Why only x86_cpu_has_work?
> >>>>
> >>>> Even reading cs->interrupt_request outside the mutex is unsafe.
> >>> All the vcpu function that access interrupt controller or peripheral state may be unsafe?
> >>> How can it work safely then?
> >>
> >> They do it inside the big QEMU lock.
> >
> > Right. Without these patches.
> > They are within the replay lock. And BQL is not covering vcpu execution with these patches.
> > Therefore RR will be ok and regular execution may encounter races?
> > It means that I missed something in Alex ideas, because he prepared the initial patches.
> 
> Yes.
> 
> >> But here you're calling cpu_has_work (via all_cpu_threads_idle) outside the lock.
> >
> > Yes, I see, but what we have to do?
> 
> I don't know.  But the idiom in these patches,
> 
> 	while(...) {
> 		lock()
> 		cond_wait()
> 		unlock()
> 	}
> 
> is unsafe as well, so the issue is more than just cpu_has_work.

Maybe it's better to omit this patch?
It seems that replay and regular execution are ok without it.

Pavel Dovgalyuk