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

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Date:   Thu, 5 Jan 2023 19:34:30 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Matthew Rosato <mjrosato@...ux.ibm.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, Jan 05, 2023 at 03:09:30PM -0700, Alex Williamson wrote:
> 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.

Bleck

It is pretty common to run the final part of 'put' from a workqueue
specifically to avoid stuff like this, eg fput does it

Maybe that is the simplest?

Jason