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SubjectRe: [PATCH 1/4] KVM: x86: fix interrupt dropping race in PIT
2016-02-03 17:23+0100, Radim Krčmář:
> A misuse of atomic operations opened a window
>
> kvm_pit_ack_irq: | pit_timer_fn:
> value = atomic_dec_return(&ps->pending); |
> | !atomic_read(&ps->pending)
> if (value < 0) atomic_inc(&ps->pending); |
>
> If ps->pending starts as 0 and we are using the discard policy, we don't
> inject any interrupt in kvm_pit_ack_irq or pit_timer_fn, leading to a
> missed PIT cycle.
>
> Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
> ---
> diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
> @@ -239,13 +239,7 @@ static void kvm_pit_ack_irq(struct kvm_irq_ack_notifier *kian)
> int value;
>
> spin_lock(&ps->inject_lock);
> - value = atomic_dec_return(&ps->pending);
> - if (value < 0)
> - /* spurious acks can be generated if, for example, the
> - * PIC is being reset. Handle it gracefully here
> - */
> - atomic_inc(&ps->pending);
> - else if (value > 0)
> + if (atomic_add_unless(&ps->pending, -1, 0))

This is not right, I'll redo the series. Sorry.

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