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

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Date:   Tue, 13 Jul 2021 14:05:33 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     Tony Krowiak <akrowiak@...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 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.

Jason