linux-kernel - Re: [PATCH] s390/vfio-ap: do not open code locks for VFIO_GROUP_NOTIFY_SET

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Message-ID: <20210713192138.GG136586@nvidia.com>
Date:   Tue, 13 Jul 2021 16:21:38 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>
Cc:     Halil Pasic <pasic@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, borntraeger@...ibm.com,
        cohuck@...hat.com, pasic@...ux.vnet.ibm.com, jjherne@...ux.ibm.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        frankja@...ux.ibm.com, david@...hat.com, imbrenda@...ux.ibm.com,
        hca@...ux.ibm.com
Subject: Re: [PATCH] s390/vfio-ap: do not open code locks for
 VFIO_GROUP_NOTIFY_SET_KVM notification

On Tue, Jul 13, 2021 at 03:04:28PM -0400, Tony Krowiak wrote:
> 
> 
> On 7/13/21 1:05 PM, Jason Gunthorpe wrote:
> > On Tue, Jul 13, 2021 at 06:45:17PM +0200, Halil Pasic wrote:
> > 
> > > Jason may give it another try to convince us that 0cc00c8d4050 only
> > > silenced lockdep, but vfio_ap remained prone to deadlocks. To my best
> > > knowledge using condition variable and a mutex is one of the well known
> > > ways to implement an rwlock.
> > The well known pattern is to use a rwsem.
> > 
> > This:
> >          wait_event_cmd(matrix_mdev->wait_for_kvm,
> >                         !matrix_mdev->kvm_busy,
> >                         mutex_unlock(&matrix_dev->lock),
> >                         mutex_lock(&matrix_dev->lock));
> > 
> > 
> > Is not really a rwsem, and is invsible to lockdep.
> 
> The lockdep splat was due to holding the matrix_dev->lock
> mutex while the kvm->lock was taken to plug the AP devices
> into the guest. The same problem would occur whether an
> rwsem or the mutex was used.

See, everytime you say 'the same problem would occur with a rwsem' you
don't get to go on and say everthing is fine if we open code a rwsem
as kvm_busy

This patch shows it works as a rwsem - look at what had to be changed
to make it so and you will find some clue to where the problems are in
kvm_busy version.

In any event, I don't care anymore - please just get this merged, to
AlexW's tree so I don't have conflicts with the rest of the ap changes
for VFIO I've got queued up.

Jason