From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757514AbcBDNgF (ORCPT ); Thu, 4 Feb 2016 08:36:05 -0500 Received: from mx1.redhat.com ([209.132.183.28]:38986 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751796AbcBDNgD (ORCPT ); Thu, 4 Feb 2016 08:36:03 -0500 Date: Thu, 4 Feb 2016 14:35:59 +0100 From: Radim =?utf-8?B?S3LEjW3DocWZ?= To: linux-kernel@vger.kernel.org Cc: kvm@vger.kernel.org, Paolo Bonzini , Yuki Shibuya Subject: Re: [PATCH 1/4] KVM: x86: fix interrupt dropping race in PIT Message-ID: <20160204133559.GB16897@potion.brq.redhat.com> References: <1454516585-28491-1-git-send-email-rkrcmar@redhat.com> <1454516585-28491-2-git-send-email-rkrcmar@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1454516585-28491-2-git-send-email-rkrcmar@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > --- > 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.