RE: [PATCH v3] vfio: fix deadlock between group lock and kvm lock

[Liu, Yi L]

> From: Tian, Kevin <kevin.tian@xxxxxxxxx>
> Sent: Friday, February 3, 2023 10:00 AM
> 
> > From: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > Sent: Friday, February 3, 2023 7:13 AM
> >
> > 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
> > > >
> > > >
> > > > 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,
> >
> 
> What about passing the lock pointer into the helper? it's still slightly
> asymmetry as the put helper doesn't carry the lock pointer but it
> could also be interpreted as if the pointer has been saved in the get
> then if it needs to be referenced by the put there is no need to pass
> it in again.

For cdev, I may modify vfio_device_get_kvm_safe() to accept
struct kvm and let its caller hold a kvm_ref_lock (field within
struct vfio_device_file). Meanwhile, the group path holds
the group->kvm_ref_lock before invoking vfio_device_get_kvm_safe().
vfio_device_get_kvm_safe() just includes the symbol get/put and
the device->kvm and put_kvm set.

Regards,
Yi Liu