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

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Message-ID: <20210628222923.21a257c8.pasic@linux.ibm.com>
Date:   Mon, 28 Jun 2021 22:29:23 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Tony Krowiak <akrowiak@...ux.ibm.com>
Cc:     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, jgg@...dia.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 use open locks during
 VFIO_GROUP_NOTIFY_SET_KVM notification

On Fri, 25 Jun 2021 18:07:58 -0400
Tony Krowiak <akrowiak@...ux.ibm.com> wrote:

What is a suitable base for this patch. I've tried the usual suspects,
but none of them worked.

> The fix to resolve a lockdep splat while handling the
> VFIO_GROUP_NOTIFY_SET_KVM event introduced a kvm_busy flag indicating that
> the vfio_ap device driver is busy setting or unsetting the KVM pointer.
> A wait queue was employed to allow functions requiring access to the KVM
> pointer to wait for the kvm_busy flag to be cleared. For the duration of
> the wait period, the mdev lock was unlocked then acquired again after the
> kvm_busy flag was cleared. This got rid of the lockdep report, but didn't
> really resolve the problem.

Can you please elaborate on the last point. You mean that we can have
circular locking even after 0cc00c8d4050, but instead of getting stuck in
on a lock we will get stuck on wait_event_cmd()? If that is it, please
state it clearly in the description, and if you can to it in the short
description.

> 
> This patch removes the the kvm_busy flag and wait queue as they are not
> necessary to resolve the lockdep splat problem. The wait queue was
> introduced to prevent changes to the matrix used to update the guest's
> AP configuration. The idea was that whenever an adapter, domain or control
> domain was being assigned to or unassigned from the matrix, the function
> would wait until the group notifier function was no longer busy with the
> KVM pointer.
> 
> The thing is, the KVM pointer value (matrix_mdev->kvm) is always set and
> cleared while holding the matrix_dev->lock mutex. The assignment and
> unassignment interfaces also lock the matrix_dev->lock mutex prior to
> checking whether the matrix_mdev->kvm pointer is set and if so, returns
> the -EBUSY error from the function. Consequently, there is no chance for
> an update to the matrix to occur while the guest's AP configuration is
> being updated.
> 
> Fixes: 0cc00c8d4050 ("s390/vfio-ap: fix circular lockdep when setting/clearing crypto masks")
> Cc: stable@...r.kernel.org
> Signed-off-by: Tony Krowiak <akrowiak@...ux.ibm.com>