linux-kernel - Re: [PATCH v3 1/1] vfio: remove VFIO_GROUP_NOTIFY_SET

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Date:   Thu, 5 Jan 2023 15:09:30 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>
Cc:     jgg@...dia.com, cohuck@...hat.com, borntraeger@...ux.ibm.com,
        jjherne@...ux.ibm.com, akrowiak@...ux.ibm.com, pasic@...ux.ibm.com,
        zhenyuw@...ux.intel.com, zhi.a.wang@...el.com, hch@...radead.org,
        intel-gfx@...ts.freedesktop.org,
        intel-gvt-dev@...ts.freedesktop.org, linux-s390@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kevin Tian <kevin.tian@...el.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v3 1/1] vfio: remove VFIO_GROUP_NOTIFY_SET_KVM

On Thu, 19 May 2022 14:33:11 -0400
Matthew Rosato <mjrosato@...ux.ibm.com> wrote:

> Rather than relying on a notifier for associating the KVM with
> the group, let's assume that the association has already been
> made prior to device_open.  The first time a device is opened
> associate the group KVM with the device.
> 
> This fixes a user-triggerable oops in GVT.

It seems this has traded an oops for a deadlock, which still exists
today in both GVT-g and vfio-ap.  These are the only vfio drivers that
care about kvm, so they make use of kvm_{get,put}_kvm(), where the
latter is called by their .close_device() callbacks.

.close_device() is called holding the group->group_lock, or at the time
of this commit group->group_rwsem.  The remaining call chain looks like
this:

kvm_put_kvm
 -> kvm_destroy_vm
  -> kvm_destroy_devices
   -> kvm_vfio_destroy
    -> kvm_vfio_file_set_kvm
     -> vfio_file_set_kvm
      -> group->group_lock/group_rwsem

Any suggestions for a fix?  Thanks,

Alex