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Date:   Tue, 25 May 2021 12:00:54 -0300
From:   Jason Gunthorpe <jgg@nvidia.com>
To:     Tony Krowiak <akrowiak@linux.ibm.com>
Cc:     linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        borntraeger@de.ibm.com, cohuck@redhat.com,
        pasic@linux.vnet.ibm.com, jjherne@linux.ibm.com,
        alex.williamson@redhat.com, kwankhede@nvidia.com,
        frankja@linux.ibm.com, david@redhat.com, imbrenda@linux.ibm.com,
        hca@linux.ibm.com
Subject: Re: [PATCH v4 2/2] s390/vfio-ap: control access to PQAP(AQIC)
 interception handler
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On Tue, May 25, 2021 at 10:59:25AM -0400, Tony Krowiak wrote:
> > But this is alot of complicated and unused code to solve a lock
> > ordering problem..
> 
> If you have a better solution, I'm all ears. I've been down this
> road a couple of times now and solving lock ordering for
> multiple asynchronous processes is not trivial. This seems like
> a reasonable solution and provides for flexibility for including
> additional hooks to handle interception of other AP instructions.

Lock ordering is very trivial. In this case you have to always hold
the hook lock before obtaining the matrix_dev lock. From what I
remember there was only one error on the set path where they were
ordered wrong

Jason