Re: [PATCH] KVM: VMX: fix lockdep warning on posted intr wakeup

[Yan Zhao]

On Fri, Mar 10, 2023 at 09:00:00AM -0800, Sean Christopherson wrote:
> On Fri, Mar 10, 2023, Yan Zhao wrote:
> > Use rcu list to break the possible circular locking dependency reported
> > by lockdep.
> > 
> > path 1, ``sysvec_kvm_posted_intr_wakeup_ipi()`` --> ``pi_wakeup_handler()``
> >          -->  ``kvm_vcpu_wake_up()`` --> ``try_to_wake_up()``,
> >          the lock sequence is
> >          &per_cpu(wakeup_vcpus_on_cpu_lock, cpu) --> &p->pi_lock.
> 
> Heh, that's an unfortunate naming collision.  It took me a bit of staring to
> realize pi_lock is a scheduler lock, not a posted interrupt lock.
me too :)

> 
> > path 2, ``schedule()`` --> ``kvm_sched_out()`` --> ``vmx_vcpu_put()`` -->
> >         ``vmx_vcpu_pi_put()`` --> ``pi_enable_wakeup_handler()``,
> >          the lock sequence is
> >          &rq->__lock --> &per_cpu(wakeup_vcpus_on_cpu_lock, cpu).
> > 
> > path 3, ``task_rq_lock()``,
> >         the lock sequence is &p->pi_lock --> &rq->__lock
> > 
> > lockdep report:
> >  Chain exists of:
> >    &p->pi_lock --> &rq->__lock --> &per_cpu(wakeup_vcpus_on_cpu_lock, cpu)
> > 
> >   Possible unsafe locking scenario:
> > 
> >         CPU0                CPU1
> >         ----                ----
> >    lock(&per_cpu(wakeup_vcpus_on_cpu_lock, cpu));
> >                             lock(&rq->__lock);
> >                             lock(&per_cpu(wakeup_vcpus_on_cpu_lock, cpu));
> >    lock(&p->pi_lock);
> > 
> >   *** DEADLOCK ***
> 
> I don't think there's a deadlock here.  pi_wakeup_handler() is called from IRQ
> context, pi_enable_wakeup_handler() disable IRQs before acquiring
> wakeup_vcpus_on_cpu_lock, and "cpu" in pi_enable_wakeup_handler() is guaranteed
> to be the current CPU, i.e. the same CPU.  So CPU0 and CPU1 can't be contending
> for the same wakeup_vcpus_on_cpu_lock in this scenario.
> 
> vmx_vcpu_pi_load() does do cross-CPU locking, but finish_task_switch() drops
> rq->__lock before invoking the sched_in notifiers.
Right. Thanks for this analysis!
But the path of pi_wakeup_handler() tells lockdep that the lock ordering
is &p->pi_lock --> &rq->__lock --> &per_cpu(wakeup_vcpus_on_cpu_lock,
cpu), so the lockdep just complains about it.

> 
> > Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> > ---
> >  arch/x86/kvm/vmx/posted_intr.c | 12 +++++-------
> >  1 file changed, 5 insertions(+), 7 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
> > index 94c38bea60e7..e3ffc45c0a7b 100644
> > --- a/arch/x86/kvm/vmx/posted_intr.c
> > +++ b/arch/x86/kvm/vmx/posted_intr.c
> > @@ -90,7 +90,7 @@ void vmx_vcpu_pi_load(struct kvm_vcpu *vcpu, int cpu)
> >  	 */
> >  	if (pi_desc->nv == POSTED_INTR_WAKEUP_VECTOR) {
> >  		raw_spin_lock(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu));
> > -		list_del(&vmx->pi_wakeup_list);
> > +		list_del_rcu(&vmx->pi_wakeup_list);
> >  		raw_spin_unlock(&per_cpu(wakeup_vcpus_on_cpu_lock, vcpu->cpu));
> 
> _If_ there is indeed a possible deadlock, there technically needs to be an explicit 
> synchonize_rcu() before freeing the vCPU.  In practice, there are probably multiple
> synchonize_rcu() calls in the destruction path, not to mention that it would take a
> minor miracle for pi_wakeup_handler() to get stalled long enough to achieve a
> use-after-free.
>
Yes, I neglected it.

Thanks for the quick and detailed review!
I will post v2 to fix it.

Yan