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

[Tony Krowiak]

On 6/28/21 4:29 PM, Halil Pasic wrote:
> On Fri, 25 Jun 2021 18:07:58 -0400
> Tony Krowiak <akrowiak@xxxxxxxxxxxxx> wrote:
>
> What is a suitable base for this patch. I've tried the usual suspects,
> but none of them worked.

I discovered what the problem is here. The patch is based on our
master branch along with the two pre-requisite patches that were
recently reviewed and are currently being merged. The two patches
of which I speak are:
* [PATCH v6 1/2] s390/vfio-ap: clean up mdev resources when remove 
callback invoked
   Message ID: <20210621155714.1198545-2-akrowiak@xxxxxxxxxxxxx>

* [PATCH v6 2/2] s390/vfio-ap: r/w lock for PQAP interception handler 
function pointer
   <20210621155714.1198545-3-akrowiak@xxxxxxxxxxxxx>

I probably should have included those along with this one.

> > 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 was in response to the following review comments made by Jason
Gunthorpe:

* Message ID: <20210525162927.GC1002214@xxxxxxxxxx>
   "... the kvm_busy should be replaced by a proper rwsem,
    don't try to open code locks like that - it just defeats lockdep
    analysis".

* Message ID: <20210527112433.GX1002214@xxxxxxxxxx>
   "Usually when people start open coding locks it is often
   because lockdep complained. Open coding a lock makes
   lockdep stop because the lockdep code
   is removed, but it doesn't fix anything. The kvm_busy
   should be replaced by a proper rwsem, don't try to
   open code locks like that - it just defeats lockdep
   analysis."

I will paraphrase and include the information from Jason's
comments in the 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@xxxxxxxxxxxxxxx
> > Signed-off-by: Tony Krowiak <akrowiak@xxxxxxxxxxxxx>