From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH 0/2] eventfd: new EFD_STATE flag Date: Mon, 11 Jan 2010 11:02:27 +0200 Message-ID: <20100111090227.GB1642@redhat.com> References: <20100106205526.GJ4001@redhat.com> <20100106222920.GB29200@redhat.com> <20100106234521.GA30515@redhat.com> <20100107064503.GA10379@redhat.com> <20100107103600.GD599@redhat.com> <20100111090127.GB7549@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Davide Libenzi , Avi Kivity , kvm@vger.kernel.org To: Gleb Natapov Return-path: Received: from mx1.redhat.com ([209.132.183.28]:59407 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751449Ab0AKJFX (ORCPT ); Mon, 11 Jan 2010 04:05:23 -0500 Content-Disposition: inline In-Reply-To: <20100111090127.GB7549@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Mon, Jan 11, 2010 at 11:01:27AM +0200, Gleb Natapov wrote: > On Thu, Jan 07, 2010 at 12:36:01PM +0200, Michael S. Tsirkin wrote: > > Or if I do it the other way: > > remove_wait_queue(irqfd->wqh, &irqfd->wait); > > -> > > eventfd_read_ctx(irqfd->eventfd, &ucnt); > > > > now, if device signals eventfd at point marked by ->, > > it will not be sent but counter will be cleared, > > so we will loose a message. > > > May be I am missing something here, but why doing it like that is a > problem? Event delivery races with interrupt masking so making masking > succeed before event delivery is OK. Event generation is asynchronous > anyway and could have happened jiffy latter anyway. > > -- > Gleb. No, event generation would only trigger a single interrupt. This race generates two interrupts for a single event. This can never happen with real hardware. eventfd_ctx_remove_wait_queue would solve this problem. -- MST