From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gregory Haskins Subject: Re: [KVM PATCH v2 2/2] kvm: use POLLHUP to close an irqfd instead of an explicit ioctl Date: Thu, 18 Jun 2009 10:03:36 -0400 Message-ID: <4A3A4938.60604@novell.com> References: <20090604124047.10544.38861.stgit@dev.haskins.net> <4A3639B2.7080006@novell.com> <20090615125439.GI6351@redhat.com> <200906181446.31373.rusty@rustcorp.com.au> <20090618064908.GB11155@redhat.com> <4A3A2C67.4030406@novell.com> <20090618122219.GE16838@redhat.com> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enigB112B91066500838AFD16119" Cc: Rusty Russell , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, avi@redhat.com, davidel@xmailserver.org, paulmck@linux.vnet.ibm.com, akpm@linux-foundation.org To: "Michael S. Tsirkin" Return-path: Received: from victor.provo.novell.com ([137.65.250.26]:47345 "EHLO victor.provo.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754334AbZFRODj (ORCPT ); Thu, 18 Jun 2009 10:03:39 -0400 In-Reply-To: <20090618122219.GE16838@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enigB112B91066500838AFD16119 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Michael S. Tsirkin wrote: > On Thu, Jun 18, 2009 at 08:00:39AM -0400, Gregory Haskins wrote: > =20 >> Michael S. Tsirkin wrote: >> =20 >> >>> BTW, Gregory, this can be used to fix the race in the design: create = a >>> thread and let it drop the module reference with module_put_and_exit.= >>> =20 >>> =20 >> I had thought of doing something like this initially too, but I think >> its racy as well. Ultimately, you need to make sure the eventfd >> callback is completely out before its safe to run, and deferring to a >> thread would not change this race. The only sane way I can see to do >> that is to have the caller infrastructure annotate the event somehow >> (either directly with a module_put(), or indirectly with some kind of >> state transition that can be tracked with something like >> synchronize_sched(). >> =20 > > Here's what one could do: create a thread for each irqfd, and increment= > module ref count, put that thread to sleep. When done with > irqfd, don't delete it and don't decrement module refcount, wake thread= > instead. thread kills irqfd and calls module_put_and_exit. > > I don't think it's racy I believe it is. How would you prevent the thread from doing the module_put_and_exit() before the eventfd callback thread is known to have exited the relevant .text section? All this talk does give me an idea, tho. Ill make a patch. =20 > =20 >>> Which will work, but I guess at this point we should ask ourselves >>> whether all the hearburn with srcu, threads and module references is >>> better than just asking the user to call and ioctl. >>> =20 >>> =20 >> I am starting to agree with you, here. :) >> >> Note one thing: the SRCU stuff is mostly orthogonal from the rest of t= he >> conversation re: the module_put() races. I only tied it into the >> current thread because the eventfd_notifier_register() thread gave me = a >> convenient way to hook some other context to do the module_put(). In >> the long term, the srcu changes are for the can_sleep() stuff. So on >> that note, lets see if I can convince Davide that the srcu stuff is no= t >> so evil before we revert the POLLHUP patches, since the module_put() f= ix >> is trivial once that is in place. >> =20 > > Can this help with DEASSIGN as well? We need it for migration. > =20 No, but afaict you do not need this for migration anyway. Migrate the GSI and re-call kvm_irqfd() on the other side. Would the fd even be relevant across a migration anyway? I would think not, but admittedly I know little about how qemu/kvm migration actually works. Regards, -Greg --------------enigB112B91066500838AFD16119 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG/MacGPG2 v2.0.11 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iEYEARECAAYFAko6STgACgkQlOSOBdgZUxn5SQCeM7JWNUqbtnzGmLWPGJnNpXgw xUwAn0LQofoohxbo0xmVq2qwe4Ikvaj3 =DPPX -----END PGP SIGNATURE----- --------------enigB112B91066500838AFD16119--