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Date:   Tue, 12 Jan 2021 17:49:35 +0100
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Matthew Rosato <mjrosato@...ux.ibm.com>
Cc:     Tony Krowiak <akrowiak@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        freude@...ux.ibm.com, borntraeger@...ibm.com, cohuck@...hat.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        fiuczy@...ux.ibm.com, frankja@...ux.ibm.com, david@...hat.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com
Subject: Re: [PATCH v13 11/15] s390/vfio-ap: implement in-use callback for
 vfio_ap driver

On Tue, 12 Jan 2021 09:14:07 -0500
Matthew Rosato <mjrosato@...ux.ibm.com> wrote:

> On 1/11/21 8:20 PM, Halil Pasic wrote:
> > On Tue, 22 Dec 2020 20:16:02 -0500
> > Tony Krowiak <akrowiak@...ux.ibm.com> wrote:
> >   
> >> Let's implement the callback to indicate when an APQN
> >> is in use by the vfio_ap device driver. The callback is
> >> invoked whenever a change to the apmask or aqmask would
> >> result in one or more queue devices being removed from the driver. The
> >> vfio_ap device driver will indicate a resource is in use
> >> if the APQN of any of the queue devices to be removed are assigned to
> >> any of the matrix mdevs under the driver's control.
> >>
> >> There is potential for a deadlock condition between the matrix_dev->lock
> >> used to lock the matrix device during assignment of adapters and domains
> >> and the ap_perms_mutex locked by the AP bus when changes are made to the
> >> sysfs apmask/aqmask attributes.
> >>
> >> Consider following scenario (courtesy of Halil Pasic):
> >> 1) apmask_store() takes ap_perms_mutex
> >> 2) assign_adapter_store() takes matrix_dev->lock
> >> 3) apmask_store() calls vfio_ap_mdev_resource_in_use() which tries
> >>     to take matrix_dev->lock
> >> 4) assign_adapter_store() calls ap_apqn_in_matrix_owned_by_def_drv
> >>     which tries to take ap_perms_mutex
> >>
> >> BANG!
> >>
> >> To resolve this issue, instead of using the mutex_lock(&matrix_dev->lock)
> >> function to lock the matrix device during assignment of an adapter or
> >> domain to a matrix_mdev as well as during the in_use callback, the
> >> mutex_trylock(&matrix_dev->lock) function will be used. If the lock is not
> >> obtained, then the assignment and in_use functions will terminate with
> >> -EBUSY.
> >>
> >> Signed-off-by: Tony Krowiak <akrowiak@...ux.ibm.com>
> >> ---
> >>   drivers/s390/crypto/vfio_ap_drv.c     |  1 +
> >>   drivers/s390/crypto/vfio_ap_ops.c     | 21 ++++++++++++++++++---
> >>   drivers/s390/crypto/vfio_ap_private.h |  2 ++
> >>   3 files changed, 21 insertions(+), 3 deletions(-)
> >>  
> > [..]  
> >>   }
> >> +
> >> +int vfio_ap_mdev_resource_in_use(unsigned long *apm, unsigned long *aqm)
> >> +{
> >> +	int ret;
> >> +
> >> +	if (!mutex_trylock(&matrix_dev->lock))
> >> +		return -EBUSY;
> >> +	ret = vfio_ap_mdev_verify_no_sharing(NULL, apm, aqm);  
> > 
> > If we detect that resources are in use, then we spit warnings to the
> > message log, right?
> > 
> > @Matt: Is your userspace tooling going to guarantee that this will never
> > happen?  
> 
> Yes, but only when using the tooling to modify apmask/aqmask.  You would 
> still be able to create such a scenario by bypassing the tooling and 
> invoking the sysfs interfaces directly.
> 
> 

Since, I suppose, the tooling is going to catch this anyway, and produce
much better feedback to the user, I believe we should be fine degrading
the severity to info or debug. 

I would prefer not producing a warning here, because I believe it is
likely to do more harm, than good (by implying a kernel problem, as I
don't think based on the message one will think that it is an userspace
problem). But if everybody else agrees, that we want a warning here, then
I can live with that as well.

Regards,
Halil

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