From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [PATCH uq/master 2/9] event_notifier: remove event_notifier_test Date: Thu, 12 Jul 2012 12:10:09 +0300 Message-ID: <4FFE9471.1060305@redhat.com> References: <1341501390-797-1-git-send-email-pbonzini@redhat.com> <1341501390-797-3-git-send-email-pbonzini@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: qemu-devel@nongnu.org, mtosatti@redhat.com, kvm@vger.kernel.org, anthony.perard@citrix.com, jan.kiszka@siemens.com, mst@redhat.com, stefano.stabellini@eu.citrix.com To: Paolo Bonzini Return-path: Received: from mx1.redhat.com ([209.132.183.28]:1687 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932960Ab2GLJK2 (ORCPT ); Thu, 12 Jul 2012 05:10:28 -0400 In-Reply-To: <1341501390-797-3-git-send-email-pbonzini@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 07/05/2012 06:16 PM, Paolo Bonzini wrote: > This is broken; since the eventfd is used in nonblocking mode there > is a race between reading and writing. > > diff --git a/event_notifier.c b/event_notifier.c > index 2b210f4..c339bfe 100644 > --- a/event_notifier.c > +++ b/event_notifier.c > @@ -51,18 +51,3 @@ int event_notifier_test_and_clear(EventNotifier *e) > int r = read(e->fd, &value, sizeof(value)); > return r == sizeof(value); > } > - > -int event_notifier_test(EventNotifier *e) > -{ > - uint64_t value; > - int r = read(e->fd, &value, sizeof(value)); > - if (r == sizeof(value)) { > - /* restore previous value. */ > - int s = write(e->fd, &value, sizeof(value)); > - /* never blocks because we use EFD_SEMAPHORE. > - * If we didn't we'd get EAGAIN on overflow > - * and we'd have to write code to ignore it. */ > - assert(s == sizeof(value)); > - } > - return r == sizeof(value); > -} I don't see the race. Mind explaining? It does however require than a poller be extra careful when reading; and the function is silly anyway. -- error compiling committee.c: too many arguments to function