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 16:20:35 +0300 Message-ID: <20090628132035.GD11866@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> 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]:53698 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752047AbZF1NVC (ORCPT ); Sun, 28 Jun 2009 09:21:02 -0400 Content-Disposition: inline In-Reply-To: <20090628125730.GB11866@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: 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 ... -- MST