Re: [patch 2/5] x86/kvm: Handle task_work on VMENTER/EXIT

[Thomas Gleixner]

On Thu, 1 Aug 2019, Oleg Nesterov wrote:
> On 08/01, Thomas Gleixner wrote:
> >
> > @@ -8172,6 +8174,10 @@ static int vcpu_run(struct kvm_vcpu *vcp
> >  			++vcpu->stat.signal_exits;
> >  			break;
> >  		}
> > +
> > +		if (notify_resume_pending())
> > +			tracehook_handle_notify_resume();
> 
> shouldn't you drop kvm->srcu before tracehook_handle_notify_resume() ?
> 
> I don't understand this code at all, but vcpu_run() does this even before
> cond_resched().

Yeah, I noticed that it's dropped around cond_resched().

My understanding is that for voluntary giving up the CPU via cond_resched()
it needs to be dropped.

For involuntary preemption (CONFIG_PREEMPT=y) it's not required as the
whole code section after preempt_enable() is fully preemptible.

Now the 1Mio$ question is whether any of the notify functions invokes
cond_resched() and whether that really matters. Paolo?

Thanks,

	tglx