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Date: Wed, 3 Feb 2016 17:23:02 +0100 From: Radim Krčmář <rkrcmar@...hat.com> To: linux-kernel@...r.kernel.org Cc: kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>, Yuki Shibuya <shibuya.yk@...s.nec.co.jp> Subject: [PATCH 1/4] KVM: x86: fix interrupt dropping race in PIT 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@...hat.com> --- arch/x86/kvm/i8254.c | 8 +------- 1 file changed, 1 insertion(+), 7 deletions(-) diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c index b0ea42b78ccd..de5f5018026f 100644 --- 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)) /* in this case, we had multiple outstanding pit interrupts * that we needed to inject. Reinject */ -- 2.7.0
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