Re: [PATCH v3 1/8] KVM: Use dedicated mutex to protect kvm_usage_count to avoid deadlock

[Sean Christopherson <seanjc@google.com>]

On Wed, Aug 14, 2024, Paolo Bonzini wrote:
> On 6/8/24 02:06, Sean Christopherson wrote:
> > Use a dedicated mutex to guard kvm_usage_count to fix a potential deadlock
> > on x86 due to a chain of locks and SRCU synchronizations.  Translating the
> > below lockdep splat, CPU1 #6 will wait on CPU0 #1, CPU0 #8 will wait on
> > CPU2 #3, and CPU2 #7 will wait on CPU1 #4 (if there's a writer, due to the
> > fairness of r/w semaphores).
> > 
> >      CPU0                     CPU1                     CPU2
> > 1   lock(&kvm->slots_lock);
> > 2                                                     lock(&vcpu->mutex);
> > 3                                                     lock(&kvm->srcu);
> > 4                            lock(cpu_hotplug_lock);
> > 5                            lock(kvm_lock);
> > 6                            lock(&kvm->slots_lock);
> > 7                                                     lock(cpu_hotplug_lock);
> > 8   sync(&kvm->srcu);
> > 
> > Note, there are likely more potential deadlocks in KVM x86, e.g. the same
> > pattern of taking cpu_hotplug_lock outside of kvm_lock likely exists with
> > __kvmclock_cpufreq_notifier()
> 
> Offhand I couldn't see any places where {,__}cpufreq_driver_target() is
> called within cpus_read_lock().  I didn't look too closely though.

Aha!  I think I finally found it and it's rather obvious now that I've found it.
I looked quite deeply on multiple occasions in the past and never found such a
case, but I could've sworn someone (Kai?) report a lockdep splat related to the
cpufreq stuff when I did the big generic hardware enabling a while back.  Of
course, I couldn't find that either :-)

Anyways...

  cpuhp_cpufreq_online()
  |
  -> cpufreq_online()
     |
     -> cpufreq_gov_performance_limits()
        |
        -> __cpufreq_driver_target()
           |
           -> __target_index()

> 
> > +``kvm_usage_count``
> > +^^^^^^^^^^^^^^^^^^^
> 
> ``kvm_usage_lock``

Good job me.