Re: [PATCH v2 01/13] perf: Ensure perf_guest_cbs aren't reloaded between !NULL check and deref

[Sean Christopherson <seanjc@google.com>]

On Sat, Aug 28, 2021, Peter Zijlstra wrote:
> On Fri, Aug 27, 2021 at 05:35:46PM -0700, Sean Christopherson wrote:
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 464917096e73..2126f6327321 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -6491,14 +6491,19 @@ struct perf_guest_info_callbacks *perf_guest_cbs;
> >  
> >  int perf_register_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
> >  {
> > -	perf_guest_cbs = cbs;
> > +	if (WARN_ON_ONCE(perf_guest_cbs))
> > +		return -EBUSY;
> > +
> > +	WRITE_ONCE(perf_guest_cbs, cbs);
> > +	synchronize_rcu();
> 
> You're waiting for all NULL users to go away? :-) IOW, we can do without
> this synchronize_rcu() call.

Doh, right.  I was thinking KVM needed to wait for in-progress NMI to exit to
ensure guest PT interrupts are handled correctly, but obviously the NMI handler
needs to exit for that CPU to get into a guest...

> >  	return 0;
> >  }
> >  EXPORT_SYMBOL_GPL(perf_register_guest_info_callbacks);
> >  
> >  int perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
> >  {
> > -	perf_guest_cbs = NULL;
> 
> 	if (WARN_ON_ONCE(perf_guest_cbs != cbs))
> 		return -EBUSY;
> 
> ?

Works for me.  I guess I'm more optimistic about people not being morons :-)