Re: possible deadlock due to irq_set_thread_affinity() calling into the scheduler (was Re: [PATCH v3 38/62] KVM: SVM: Take and hold ir_list_lock across IRTE updates in IOMMU)

Paolo Bonzini
2026-01-21T18:13:56Z

Sorry, not sure how the previous email ended up encrypted.

On 1/8/26 22:53, Thomas Gleixner wrote:
> On Thu, Jan 08 2026 at 22:28, Thomas Gleixner wrote:
>> On Mon, Dec 22 2025 at 15:09, Paolo Bonzini wrote:
>>> Of the three, the most sketchy is (a); notably, __setup_irq() calls
>>> wake_up_process outside desc->lock. Therefore I'd like so much to treat
>>> it as a kernel/irq/ bug; and the simplest (perhaps too simple...) fix is
>>
>> It's not more sketchy than VIRT assuming that it can do what it wants
>> under rq->lock. 🙂
>
> And just for the record, that's not the only place in the irq core which
> has that lock chain.
>
> irq_set_affinity_locked() // invoked with desc::lock held
> if (desc->affinity_notify)
> schedule_work() // Ends up taking rq::lock
>
> and that's the case since cd7eab44e994 ("genirq: Add IRQ affinity
> notifiers"), which was added 15 years ago.
>
> Are you still claiming that this is a kernel/irq bug?

Not really, I did say I'd like to treat it as a kernel/irq bug...
but certainly didn't have hopes high enough to "claim" that.
I do think that it's ugly to have locks that are internal,
non-leaf and held around callbacks; but people smarter than
me have thought about it and you can't call it a bug anyway.

For x86/AMD we have a way to fix it, so that part is not a problem.

For the call(*) to irq_set_affinity() in arch/arm64/kvm/'s
vgic_v4_load() I think it can be solved as well.
kvm_make_request(KVM_REQ_RELOAD_GICv4) will delay vgic_v4_load()
to a safe spot, so just cache the previous smp_processor_id() and,
if it is different, do the kvm_make_request() and return instead
of calling irq_set_affinity().

vgic_v3_load() is the only place that calls it from the preempt
notifier, so this behavior can be tied to a "bool delay_set_affinity"
argument to vgic_v4_load() or placed in a different function.

Marc/Oliver, does that sound doable?

Paolo

(*) kvm_sched_in() preempt notifier -> kvm_arch_vcpu_load() ->
kvm_vgic_load() -> vgic_v3_load() -> vgic_v4_load()