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Date:   Thu, 19 Jan 2023 12:05:13 -0700
From:   Alex Williamson <alex.williamson@redhat.com>
To:     "Tian, Kevin" <kevin.tian@intel.com>
Cc:     Matthew Rosato <mjrosato@linux.ibm.com>,
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Subject: Re: [PATCH v4] vfio: fix potential deadlock on vfio group lock
Message-ID: <20230119120513.3976cda7.alex.williamson@redhat.com>
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On Thu, 19 Jan 2023 03:43:36 +0000
"Tian, Kevin" <kevin.tian@intel.com> wrote:

> > From: Matthew Rosato <mjrosato@linux.ibm.com>
> > Sent: Wednesday, January 18, 2023 10:56 PM
> >=20
> > On 1/18/23 4:03 AM, Tian, Kevin wrote: =20
> > >> From: Alex Williamson
> > >> Sent: Wednesday, January 18, 2023 5:23 AM
> > >>
> > >> On Fri, 13 Jan 2023 19:03:51 -0500
> > >> Matthew Rosato <mjrosato@linux.ibm.com> wrote:
> > >> =20
> > >>>  void vfio_device_group_close(struct vfio_device *device)
> > >>>  {
> > >>> +	void (*put_kvm)(struct kvm *kvm);
> > >>> +	struct kvm *kvm;
> > >>> +
> > >>>  	mutex_lock(&device->group->group_lock);
> > >>> +	kvm =3D device->kvm;
> > >>> +	put_kvm =3D device->put_kvm;
> > >>>  	vfio_device_close(device, device->group->iommufd);
> > >>> +	if (kvm =3D=3D device->kvm)
> > >>> +		kvm =3D NULL; =20
> > >>
> > >> Hmm, so we're using whether the device->kvm pointer gets cleared in
> > >> last_close to detect whether we should put the kvm reference.  That'=
s a
> > >> bit obscure.  Our get and put is also asymmetric.
> > >>
> > >> Did we decide that we couldn't do this via a schedule_work() from the
> > >> last_close function, ie. implementing our own version of an async pu=
t?
> > >> It seems like that potentially has a cleaner implementation, symmetr=
ic
> > >> call points, handling all the storing and clearing of kvm related
> > >> pointers within the get/put wrappers, passing only a vfio_device to =
the
> > >> put wrapper, using the "vfio_device_" prefix for both.  Potentially
> > >> we'd just want an unconditional flush outside of lock here for
> > >> deterministic release.
> > >>
> > >> What's the downside?  Thanks,
> > >> =20
> > >
> > > btw I guess this can be also fixed by Yi's work here:
> > >
> > > https://lore.kernel.org/kvm/20230117134942.101112-6-yi.l.liu@intel.co=
m/
> > >
> > > with set_kvm(NULL) moved to the release callback of kvm_vfio device,
> > > such circular lock dependency can be avoided too. =20
> >=20
> > Oh, interesting...  It seems to me that this would eliminate the report=
ed call
> > chain altogether:
> >=20
> > kvm_put_kvm =20
> >  -> kvm_destroy_vm
> >   -> kvm_destroy_devices
> >    -> kvm_vfio_destroy (starting here -- this would no longer be execut=
ed)
> >     -> kvm_vfio_file_set_kvm
> >      -> vfio_file_set_kvm
> >       -> group->group_lock/group_rwsem =20
> >=20
> > because kvm_destroy_devices now can't end up calling kvm_vfio_destroy
> > and friends, it won't try and acquire the group lock a 2nd time making a
> > kvm_put_kvm while the group lock is held OK to do.  The vfio_file_set_k=
vm
> > call will now always come from a separate thread of execution,
> > kvm_vfio_group_add, kvm_vfio_group_del or the release thread:
> >=20
> > kvm_device_release (where the group->group_lock would not be held since
> > vfio does not trigger closing of the kvm fd) =20
> >  -> kvm_vfio_destroy (or, kvm_vfio_release)
> >   -> kvm_vfio_file_set_kvm
> >    -> vfio_file_set_kvm
> >     -> group->group_lock/group_rwsem =20
>=20
> Yes, that's my point. If Alex/Jason are also OK with it probably Yi can
> send that patch separately as a fix to this issue. It's much simpler. =F0=
=9F=98=8A

If we can extract that flow separate from the cdev refactoring, ideally
something that matches the stable kernel backport rules, then that
sounds like the preferred solution.  Thanks,

Alex