Re: [PATCH 05/13] kvm/vfio: Provide struct kvm_device_ops::release() insted of ::destroy()

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Jan 17, 2023 at 05:49:34AM -0800, Yi Liu wrote:
> This is to avoid a circular refcount problem between the kvm struct and
> the device file. KVM modules holds device/group file reference when the
> device/group is added and releases it per removal or the last kvm reference
> is released. This reference model is ok for the group since there is no
> kvm reference in the group paths.
> 
> But it is a problem for device file since the vfio devices may get kvm
> reference in the device open path and put it in the device file release.
> e.g. Intel kvmgt. This would result in a circular issue since the kvm
> side won't put the device file reference if kvm reference is not 0, while
> the vfio device side needs to put kvm reference in the release callback.
> 
> To solve this problem for device file, let vfio provide release() which
> would be called once kvm file is closed, it won't depend on the last kvm
> reference. Hence avoid circular refcount problem.
> 
> Suggested-by: Kevin Tian <kevin.tian@intel.com>
> Signed-off-by: Yi Liu <yi.l.liu@intel.com>
> ---
>  virt/kvm/vfio.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>

From Alex's remarks please revise the commit message and add a Fixes
line of some kind that this solves the deadlock Matthew was working
on, and send it stand alone right away

Jason