From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1761803AbZFXTmy (ORCPT ); Wed, 24 Jun 2009 15:42:54 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753909AbZFXTmr (ORCPT ); Wed, 24 Jun 2009 15:42:47 -0400 Received: from victor.provo.novell.com ([137.65.250.26]:35939 "EHLO victor.provo.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753287AbZFXTmq (ORCPT ); Wed, 24 Jun 2009 15:42:46 -0400 Message-ID: <4A4281AA.4040708@novell.com> Date: Wed, 24 Jun 2009 15:42:34 -0400 From: Gregory Haskins User-Agent: Thunderbird 2.0.0.22 (Macintosh/20090605) MIME-Version: 1.0 To: "linux-kernel@vger.kernel.org" CC: dhowells@redhat.com Subject: Re: [RFC PATCH] slow-work: add (module*)work->owner to fix races with module clients References: <4A427F83.8010404@novell.com> In-Reply-To: <4A427F83.8010404@novell.com> X-Enigmail-Version: 0.95.7 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="------------enigA852F9B911E2D02A0A661FEB" Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This is an OpenPGP/MIME signed message (RFC 2440 and 3156) --------------enigA852F9B911E2D02A0A661FEB Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: quoted-printable Gregory Haskins wrote: > (Applies to Linus' git master:626f380d) > > Hi All, > I found this while working on KVM. I actually posted this patch with= > a KVM > series yesterday and standalone earlier today, but neither seems to hav= e > made it to the lists. I suspect there is an issue with git-mail/postfi= x > on my system. > > I digress. This is a repost with the patch by itself, and rebased to > Linus' tree instead of kvm.git. Apologies if the system finally > corrects itself and the others show up. > > Thoughts? > > Regards, > -Greg > > ----------------------------- > > slow-work: add (module*)work->owner to fix races with module clients > > The slow_work facility was designed to use reference counting instead o= f > barriers for synchronization. The reference counting mechanism is > implemented as a vtable op (->get_ref, ->put_ref) callback. This is > problematic for module use of the slow_work facility because it is > impossible > to synchronize against the .text installed in the callbacks: There is > no way to ensure that the slow-work threads have completely exited the > .text in question and rmmod may yank it out from under the slow_work th= read. > > This patch attempts to address this issue by transparently mapping "str= uct > module* owner" to the slow_work item, and maintaining a module referenc= e > count coincident with the more externally visible reference count. Sin= ce > the slow_work facility is resident in kernel, it should be a race-free > location to issue a module_put() call. This will ensure that modules > can properly cleanup before exiting. > > A module_get()/module_put() pair on slow_work_enqueue() and the subsequ= ent > dequeue technically adds the overhead of the atomic operations for ever= y > work item scheduled. However, slow_work is designed for deferring > relatively long-running and/or sleepy tasks to begin with, so this > overhead will hopefully be negligible. > > Signed-off-by: Gregory Haskins > CC: David Howells > --- > > include/linux/slow-work.h | 4 ++++ > kernel/slow-work.c | 6 ++++++ > 2 files changed, 10 insertions(+), 0 deletions(-) > > diff --git a/include/linux/slow-work.h b/include/linux/slow-work.h > index b65c888..9f48dab 100644 > --- a/include/linux/slow-work.h > +++ b/include/linux/slow-work.h > @@ -17,6 +17,7 @@ > #ifdef CONFIG_SLOW_WORK > =20 > #include > +#include > =20 > struct slow_work; > =20 > @@ -42,6 +43,7 @@ struct slow_work_ops { > * queued > */ > struct slow_work { > + struct module *owner; > unsigned long flags; > #define SLOW_WORK_PENDING 0 /* item pending (further) execution = */ > #define SLOW_WORK_EXECUTING 1 /* item currently executing */ > @@ -61,6 +63,7 @@ struct slow_work { > static inline void slow_work_init(struct slow_work *work, > const struct slow_work_ops *ops) > { > + work->owner =3D THIS_MODULE; > work->flags =3D 0; > work->ops =3D ops; > INIT_LIST_HEAD(&work->link); > @@ -78,6 +81,7 @@ static inline void slow_work_init(struct slow_work *w= ork, > static inline void vslow_work_init(struct slow_work *work, > const struct slow_work_ops *ops) > { > + work->owner =3D THIS_MODULE; > work->flags =3D 1 << SLOW_WORK_VERY_SLOW; > work->ops =3D ops; > INIT_LIST_HEAD(&work->link); > diff --git a/kernel/slow-work.c b/kernel/slow-work.c > index 09d7519..1dc3486 100644 > --- a/kernel/slow-work.c > +++ b/kernel/slow-work.c > @@ -220,6 +220,8 @@ static bool slow_work_execute(void) > } > =20 > work->ops->put_ref(work); > =20 On this front: I also wonder if this put_ref is racing since we cannot guarantee pointer stability if the object is kfree'd as a result of dropping the last ref. I do not know enough about compilers to say whether work or work->ops invalidation would cause problems with the call-return, but it seems dangerous at best. An alternative might be to copy the put_ref pointer prior to the call. Something like slowwork_putref_t put_ref =3D work->ops->put_ref; .... put_ref(work); might be better. However, I am not sure if it really matters so I did not address this issue yet. -Greg > + barrier(); /* ensure that put_ref is not re-ordered with module_pu= t */ > + module_put(work->owner); > return true; > =20 > auto_requeue: > @@ -299,6 +301,8 @@ int slow_work_enqueue(struct slow_work *work) > if (test_bit(SLOW_WORK_EXECUTING, &work->flags)) { > set_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags); > } else { > + if (!try_module_get(work->owner)) > + goto cant_get_mod; > if (work->ops->get_ref(work) < 0) > goto cant_get_ref; > if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags)) > @@ -313,6 +317,8 @@ int slow_work_enqueue(struct slow_work *work) > return 0; > =20 > cant_get_ref: > + module_put(work->owner); > +cant_get_mod: > spin_unlock_irqrestore(&slow_work_queue_lock, flags); > return -EAGAIN; > } > > > =20 --------------enigA852F9B911E2D02A0A661FEB 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 iEYEARECAAYFAkpCgaoACgkQlOSOBdgZUxlcuQCfdDlGNItiKc4iFyFX40BmvpCI yNQAnj7APmgDBPNfD8x7VN07O/0FKrdN =Gnsm -----END PGP SIGNATURE----- --------------enigA852F9B911E2D02A0A661FEB--