On Thu, 2 Feb 2023 23:04:10 +0000
"Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote:
> > From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > Sent: Friday, February 3, 2023 3:42 AM
> >
> > On Thu, 2 Feb 2023 11:24:42 -0500
> > Matthew Rosato <mjrosato@xxxxxxxxxxxxx> wrote:
> >
> > > After 51cdc8bc120e, we have another deadlock scenario between the
> > > kvm->lock and the vfio group_lock with two different codepaths acquiring
> > > the locks in different order. Specifically in vfio_open_device, vfio
> > > holds the vfio group_lock when issuing device->ops->open_device but
> > some
> > > drivers (like vfio-ap) need to acquire kvm->lock during their open_device
> > > routine; Meanwhile, kvm_vfio_release will acquire the kvm->lock first
> > > before calling vfio_file_set_kvm which will acquire the vfio group_lock.
> > >
> > > To resolve this, let's remove the need for the vfio group_lock from the
> > > kvm_vfio_release codepath. This is done by introducing a new spinlock to
> > > protect modifications to the vfio group kvm pointer, and acquiring a kvm
> > > ref from within vfio while holding this spinlock, with the reference held
> > > until the last close for the device in question.
> > >
> > > Fixes: 51cdc8bc120e ("kvm/vfio: Fix potential deadlock on vfio group_lock")
> > > Reported-by: Anthony Krowiak <akrowiak@xxxxxxxxxxxxx>
> > > Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> > > Signed-off-by: Matthew Rosato <mjrosato@xxxxxxxxxxxxx>
> > > ---
> > > Changes from v2:
> > > * Relocate the new functions back to vfio_main and externalize to call
> > > from group (Kevin) since cdev will need this too
> > > * s/vfio_kvm_*_kvm/vfio_device_*_kvm/ and only pass device as input.
> > > Handle new kvm_ref_lock directly inside vfio_device_get_kvm (Alex)
> > > * Add assert_lockdep_held for dev_set lock (Alex)
> > > * Internalize error paths for vfio_device_get_kvm_safe and now return
> > > void - either device->kvm is set with a ref taken or is NULL (Alex)
> > > * Other flow suggestions to make the call path cleaner - Thanks! (Alex)
> > > * Can't pass group->kvm to vfio_device_open, as it references the value
> > > outside of new lock. Pass device->kvm to minimize changes in this
> > > fix (Alex, Yi)
> > > Changes from v1:
> > > * use spin_lock instead of spin_lock_irqsave (Jason)
> > > * clear device->kvm_put as part of vfio_kvm_put_kvm (Yi)
> > > * Re-arrange code to avoid referencing the group contents from within
> > > vfio_main (Kevin) which meant moving most of the code in this patch
> > > to group.c along with getting/dropping of the dev_set lock
> > > ---
> > > drivers/vfio/group.c | 32 ++++++++++++++----
> > > drivers/vfio/vfio.h | 14 ++++++++
> > > drivers/vfio/vfio_main.c | 70 ++++++++++++++++++++++++++++++++++++--
> > --
> > > include/linux/vfio.h | 2 +-
> > > 4 files changed, 103 insertions(+), 15 deletions(-)
> >
> > LGTM. I'm not sure moving the functions to vfio_main really buys us
> > anything since we're making so much use of group fields. The cdev
> > approach will necessarily be different, so the bulk of the get code will
> > likely need to move back to group.c anyway.
> >
>
> well my last comment was based on Matthew's v2 where the get code
> gets a kvm passed in instead of implicitly retrieving group ref_lock
> internally. In that case the get/put helpers only contain device logic
> thus fit in vfio_main.c.
>
> with v3 then they have to be in group.c since we don't want to use
> group fields in vfio_main.c.
>
> but I still think v2 of the helpers is slightly better. The only difference
> between cdev and group when handling this race is using different
> ref_lock. the symbol get/put part is exactly same. So even if we
> merge v3 like this, very likely Yi has to change it back to v2 style
> to share the get/put helpers while just leaving the ref_lock part
> handled differently between the two path.
I'm not really a fan of the asymmetry of the v2 version where the get
helper needs to be called under the new kvm_ref_lock, but the put
helper does not. Having the get helper handle that makes the caller
much cleaner. Thanks,
Alex
