Re: [RFC PATCH v3 5/6] KVM: SVM: Add support for AMD IOMMU Guest APIC Physical Processor Interrupt (GAPPI)

On Mon, Jul 13, 2026, Sairaj Kodilkar wrote:
> On 7/13/2026 4:41 PM, [email protected] wrote:
> > [Severity: High]
> > Does this lock ordering create an ABBA deadlock?
> > 
> > When a vCPU is preempted, the scheduler executes kvm_sched_out() while holding
> > the runqueue lock (rq_lock). KVM then calls __avic_vcpu_put(), which in turn
> > calls avic_add_vcpu_to_gappi_wakeup_list(). This acquires the
> > gappi_vcpu_wakeup_list_lock, establishing an ordering of rq_lock ->
> > gappi_vcpu_wakeup_list_lock.
> > 
> > Conversely, when a GAPPI interrupt arrives, avic_gappi_wakeup_handler()
> > acquires gappi_vcpu_wakeup_list_lock and calls kvm_vcpu_wake_up(). The wakeup
> > path then attempts to acquire the target task's runqueue lock, establishing
> > the reverse order: gappi_vcpu_wakeup_list_lock -> rq_lock.
> > 
> > This inversion will likely trigger a lockdep ABBA deadlock splat and violates
> > the scheduler locking hierarchy.
> 
> For this condition to exist, Following two conditions must be true.
> 1. avic_vcpu_put() and gappi interrupt handler() must be on same
> physical CPU.
> 2. GAPPI must preempt the __avic_vcpu_put() once it has acquired
> gappi_vcpu_wakeup_list_lock().
> 
> The second condition is not possible since __avic_vcpu_put() disables
> the interrupt while aquiring the ir_list lock.
> 
> Hence ABBA deadlock is not possible

I wouldn't be surprised if you need to tell lockdep that, e.g. see commit
c0b8dcabb2cd ("KVM: VMX: Use separate subclasses for PI wakeup lock to squash
false positive").