From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [KVM PATCH v5 3/4] KVM: Fix races in irqfd using new eventfd_kref_get interface Date: Sun, 28 Jun 2009 22:07:10 +0300 Message-ID: <20090628190710.GB14136@redhat.com> References: <20090625132441.26748.641.stgit@dev.haskins.net> <20090625132826.26748.15607.stgit@dev.haskins.net> <20090628114846.GA11764@redhat.com> <4A4767C2.3010503@novell.com> <20090628125612.GA11866@redhat.com> <20090628125730.GB11866@redhat.com> <20090628132035.GD11866@redhat.com> <4A479A23.1010804@novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: kvm@vger.kernel.org, avi@redhat.com To: Gregory Haskins Return-path: Received: from mx2.redhat.com ([66.187.237.31]:57018 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752028AbZF1THh (ORCPT ); Sun, 28 Jun 2009 15:07:37 -0400 Content-Disposition: inline In-Reply-To: <4A479A23.1010804@novell.com> Sender: kvm-owner@vger.kernel.org List-ID: On Sun, Jun 28, 2009 at 12:28:19PM -0400, Gregory Haskins wrote: > Michael S. Tsirkin wrote: > > On Sun, Jun 28, 2009 at 03:57:30PM +0300, Michael S. Tsirkin wrote: > > > >> On Sun, Jun 28, 2009 at 03:56:12PM +0300, Michael S. Tsirkin wrote: > >> > >>> On Sun, Jun 28, 2009 at 08:53:22AM -0400, Gregory Haskins wrote: > >>> > >>>> Michael S. Tsirkin wrote: > >>>> > >>>>> On Thu, Jun 25, 2009 at 09:28:27AM -0400, Gregory Haskins wrote: > >>>>> > >>>>> > >>>>>> eventfd currently emits a POLLHUP wakeup on f_ops->release() to generate a > >>>>>> "release" callback. This lets eventfd clients know if the eventfd is about > >>>>>> to go away and is very useful particularly for in-kernel clients. However, > >>>>>> until recently it is not possible to use this feature of eventfd in a > >>>>>> race-free way. This patch utilizes a new eventfd interface to rectify > >>>>>> the problem. > >>>>>> > >>>>>> Note that one final race is known to exist: the slow-work thread may race > >>>>>> with module removal. We are currently working with slow-work upstream > >>>>>> to fix this issue as well. Since the code prior to this patch also > >>>>>> races with module_put(), we are not making anything worse, but rather > >>>>>> shifting the cause of the race. Once the slow-work code is patched we > >>>>>> will be fixing the last remaining issue. > >>>>>> > >>>>>> > >>>>> By the way, why are we using slow-work here? Wouldn't a regular > >>>>> workqueue do just as well, with less code, and avoid the race? > >>>>> > >>>>> > >>>>> > >>>> I believe it will cause a problem if you do a "flush_work()" from inside > >>>> a work-item. I could be wrong, of course, but it looks like a recipe to > >>>> deadlock. > >>>> > >>>> -Greg > >>>> > >>>> > >>> Sure, but the idea is to only flush on kvm close, never from work item. > >>> > >> To clarify, you don't flush slow works from a work-item, > >> so you shouldn't need to flush workqueue either. > >> > > > > I guess my question is - why is slow work different? It's still > > a thread pool underneath ... > > > > > Its not interdependent. Flush-work blocks the thread..if the thread > happens to be the work-queue thread you may deadlock preventing it from > processing further jobs like the inject. In reality it shouldnt be > possible, but its just a bad idea to assume its ok. > Slow work, on the > other hand, will just make a new thread. > > -Greg > But if you create your own workqueue, and all you do there is destroy irqfds, things are ok I think. Right?