Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755336AbZFBPQE (ORCPT ); Tue, 2 Jun 2009 11:16:04 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755371AbZFBPPl (ORCPT ); Tue, 2 Jun 2009 11:15:41 -0400 Received: from victor.provo.novell.com ([137.65.250.26]:48508 "EHLO victor.provo.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754549AbZFBPPj (ORCPT ); Tue, 2 Jun 2009 11:15:39 -0400 From: Gregory Haskins Subject: [KVM-RFC PATCH 1/2] eventfd: send POLLHUP on f_ops->release To: kvm@vger.kernel.org Cc: linux-kernel@vger.kernel.org, avi@redhat.com, davidel@xmailserver.org, mst@redhat.com, paulmck@linux.vnet.ibm.com Date: Tue, 02 Jun 2009 11:15:33 -0400 Message-ID: <20090602151533.29746.1533.stgit@dev.haskins.net> In-Reply-To: <20090602151135.29746.91320.stgit@dev.haskins.net> References: <20090602151135.29746.91320.stgit@dev.haskins.net> User-Agent: StGIT/0.14.3 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 5131 Lines: 139 From: Davide Libenzi Return-path: Received: from novell.com ([::ffff:130.57.1.153]) by soto.provo.novell.com with ESMTP; Wed, 27 May 2009 13:35:04 -0600 Received: from x35.xmailserver.org not authenticated [64.71.152.41] by novell.com with M+ Extreme Email Engine 2008.3.release via secured & encrypted transport (TLS); Wed, 27 May 2009 13:35:04 -0600 X-MailFrom: davidel@xmailserver.org X-AuthUser: davidel@xmailserver.org Received: from makko.or.mcafeemobile.com by x35.xmailserver.org with [XMail 1.26 ESMTP Server] id for from ; Wed, 27 May 2009 15:34:01 -0400 Date: Wed, 27 May 2009 12:28:57 -0700 (PDT) From: Davide Libenzi X-X-Sender: davide@makko.or.mcafeemobile.com To: "Michael S. Tsirkin" cc: Gregory Haskins , kvm@vger.kernel.org, Linux Kernel Mailing List , avi@redhat.com, mtosatti@redhat.com Subject: Re: [KVM PATCH v10] kvm: add support for irqfd In-Reply-To: <20090527184106.GA18463@redhat.com> Message-ID: References: <20090520142234.22285.72274.stgit@dev.haskins.net> <20090527130447.GA11643@redhat.com> <4A1D48FA.9080403@novell.com> <20090527184106.GA18463@redhat.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 On Wed, 27 May 2009, Michael S. Tsirkin wrote: > On Wed, May 27, 2009 at 10:06:50AM -0400, Gregory Haskins wrote: > > Michael S. Tsirkin wrote: > > > On Wed, May 20, 2009 at 10:30:49AM -0400, Gregory Haskins wrote: > > > > > >> +static int > > >> +kvm_assign_irqfd(struct kvm *kvm, int fd, int gsi) > > >> +{ > > >> + struct _irqfd *irqfd; > > >> + struct file *file = NULL; > > >> + int ret; > > >> + > > >> + irqfd = kzalloc(sizeof(*irqfd), GFP_KERNEL); > > >> + if (!irqfd) > > >> + return -ENOMEM; > > >> + > > >> + irqfd->kvm = kvm; > > >> + irqfd->gsi = gsi; > > >> + INIT_LIST_HEAD(&irqfd->list); > > >> + INIT_WORK(&irqfd->work, irqfd_inject); > > >> + > > >> + /* > > >> + * Embed the file* lifetime in the irqfd. > > >> + */ > > >> + file = fget(fd); > > >> + if (IS_ERR(file)) { > > >> + ret = PTR_ERR(file); > > >> + goto fail; > > >> + } > > >> > > > > > > So we get a reference to a file, and unless the user is nice to us, it > > > will only be dropped when kvm char device file is closed? > > > I think this will deadlock if the fd in question is the open kvm char device. > > > > > > > > > > > Hmm...I hadn't considered this possibility, though I am not sure if it > > would cause a deadlock in the pattern you suggest. It seems more like > > it would result in, at worst, an extra reference to itself (and thus a > > leak) rather than a deadlock... > > > > I digress. In either case, perhaps I should s/fget/eventfd_fget to at > > least limit the type of fd to eventfd. I was trying to be "slick" by > > not needing the eventfd_fget() exported, but I am going to need to > > export it later anyway for iosignalfd, so its probably a moot point. > > > > Thanks Michael, > > -Greg > > > > This only works as long as eventfd does not do fget on some fd as well. > Which it does not do now, and may never do - but we create a fragile > system this way. > > I think it's really wrong, fundamentally, to keep a reference to a > file until another file is closed, unless you are code under fs/. > We will get nasty circular references sooner or later. > > Isn't the real reason we use fd to be able to support the same interface > on top of both kvm and lguest? > And if so, wouldn't some kind of bus be a better solution? Another solution, that I proposed in the past, is having irqfd hold no references to the eventfd. It's just register (holding an eventfd-get()) for events (in the way that currently does), notice the POLLHUP, unchain from it, and propagate the eventfd-close event to whatever the irqfd logic is supposed to. - Davide Signed-off-by: Gregory Haskins --- fs/eventfd.c | 10 +++++++++- 1 files changed, 9 insertions(+), 1 deletions(-) diff --git a/fs/eventfd.c b/fs/eventfd.c index 3f0e197..72f5f8d 100644 --- a/fs/eventfd.c +++ b/fs/eventfd.c @@ -61,7 +61,15 @@ EXPORT_SYMBOL_GPL(eventfd_signal); static int eventfd_release(struct inode *inode, struct file *file) { - kfree(file->private_data); + struct eventfd_ctx *ctx = file->private_data; + + /* + * No need to hold the lock here, since we are on the file cleanup + * path and the ones still attached to the wait queue will be + * serialized by wake_up_locked_poll(). + */ + wake_up_locked_poll(&ctx->wqh, POLLHUP); + kfree(ctx); return 0; } -- 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/