Re: [PATCH] s390/vfio-ap: do not open code locks for VFIO_GROUP_NOTIFY_SET_KVM notification

[Jason Gunthorpe]

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