Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760259AbZFSWxb (ORCPT ); Fri, 19 Jun 2009 18:53:31 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759531AbZFSWxU (ORCPT ); Fri, 19 Jun 2009 18:53:20 -0400 Received: from x35.xmailserver.org ([64.71.152.41]:51465 "EHLO x35.xmailserver.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758198AbZFSWxT (ORCPT ); Fri, 19 Jun 2009 18:53:19 -0400 X-AuthUser: davidel@xmailserver.org Date: Fri, 19 Jun 2009 15:47:11 -0700 (PDT) From: Davide Libenzi X-X-Sender: davide@makko.or.mcafeemobile.com To: Gregory Haskins cc: mst@redhat.com, kvm@vger.kernel.org, Linux Kernel Mailing List , avi@redhat.com, paulmck@linux.vnet.ibm.com, Ingo Molnar Subject: Re: [PATCH 3/3] eventfd: add internal reference counting to fix notifier race conditions In-Reply-To: Message-ID: References: <20090619183534.31118.30934.stgit@dev.haskins.net> <20090619185138.31118.14916.stgit@dev.haskins.net> <4A3C004B.8010706@novell.com> <4A3C07FF.3000406@novell.com> User-Agent: Alpine 1.10 (DEB 962 2008-03-14) X-GPG-FINGRPRINT: CFAE 5BEE FD36 F65E E640 56FE 0974 BF23 270F 474E X-GPG-PUBLIC_KEY: http://www.xmailserver.org/davidel.asc MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1709 Lines: 51 On Fri, 19 Jun 2009, Davide Libenzi wrote: > On Fri, 19 Jun 2009, Gregory Haskins wrote: > > > I am fairly confident it is not that simple after having thought about > > this issue over the last few days. But I've been wrong in the past. > > Propose a patch and I will review it for races/correctness, if you > > like. Perhaps a combination of that plus your asymmetrical locking > > scheme would work. One of the challenges you will hit is avoiding ABBA > > between your "get" lock and the wqh, but good luck! > > A patch for what? The eventfd patch is a one-liner. > It seems hard to believe that the thing cannot be handled on your side. > Once the wake_up_locked() is turned into a wake_up(), what other races are > there? AFAICS, the IRQfd code simply registers the callback to ->poll() and waits for two events. In the POLLIN event, you schedule_work(&irqfd->inject) and there are no races there AFAICS (you basically do not care of anything eventfd memory related at all). For POLLHUP, you do: spin_lock(irqfd->slock); if (irqfd->wqh) schedule_work(&irqfd->inject); irqfd->wqh = NULL; spin_unlock(irqfd->slock); In your work function you notice the POLLHUP condition and take proper action (dunno what it is in your case). In your kvm_irqfd_release() function: spin_lock(irqfd->slock); if (irqfd->wqh) remove_wait_queue(irqfd->wqh, &irqfd->wait); irqfd->wqh = NULL; spin_unlock(irqfd->slock); Any races in there? - Davide -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/