From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757647AbcBCQYF (ORCPT ); Wed, 3 Feb 2016 11:24:05 -0500 Received: from mx1.redhat.com ([209.132.183.28]:44391 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757160AbcBCQYB (ORCPT ); Wed, 3 Feb 2016 11:24:01 -0500 From: =?UTF-8?q?Radim=20Kr=C4=8Dm=C3=A1=C5=99?= To: linux-kernel@vger.kernel.org Cc: kvm@vger.kernel.org, Paolo Bonzini , Yuki Shibuya Subject: [PATCH 1/4] KVM: x86: fix interrupt dropping race in PIT Date: Wed, 3 Feb 2016 17:23:02 +0100 Message-Id: <1454516585-28491-2-git-send-email-rkrcmar@redhat.com> In-Reply-To: <1454516585-28491-1-git-send-email-rkrcmar@redhat.com> References: <1454516585-28491-1-git-send-email-rkrcmar@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- 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