[PATCH v3 01/16] perf: Ensure perf_guest_cbs aren't reloaded between !NULL check and deref

[Sean Christopherson]

On Wed, Nov 10, 2021, Paolo Bonzini wrote:
> On 11/4/21 15:18, Sean Christopherson wrote:
> > If I'm interpeting Paolo's suggestion
> > correctly, he's pointing out that oustanding stores to the function pointers in
> > @cbs need to complete before assigning a non-NULL pointer to perf_guest_cbs,
> > otherwise a perf event handler may see a valid pointer with half-baked callbacks.
> > 
> > I think smp_store_release() with a comment would be appropriate, assuming my
> > above interpretation is correct.
> > 
> 
> Yes, exactly.  It should even be rcu_assign_pointer(), matching the
> synchronize_rcu()

And perf_guest_cbs should be tagged __rcu and accessed accordingly.  Which is
effectively what this version (poorly) implemented with a homebrewed mix of
{READ,WRITE}_ONCE, lockdep(), and synchronize_rcu().

> in patch 1 (and the change can be done in patch 1, too).

Ya, the change needs to go into patch 1.