linux-kernel - Re: [PATCH 2/2] s390/vfio-ap: replace open coded locks for VFIO_GROUP_NOTIFY_SET

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Message-ID: <20210727004329.3bcc7d4f.pasic@linux.ibm.com>
Date:   Tue, 27 Jul 2021 00:43:29 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Tony Krowiak <akrowiak@...ux.ibm.com>, 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,
        alex.williamson@...hat.com, kwankhede@...dia.com, david@...hat.com
Subject: Re: [PATCH 2/2] s390/vfio-ap: replace open coded locks for
 VFIO_GROUP_NOTIFY_SET_KVM notification

On Mon, 26 Jul 2021 19:03:17 -0300
Jason Gunthorpe <jgg@...dia.com> wrote:

> > You may end up with open and close running interleaved. What I'
> > trying to say is, to my best knowledge, normally there is no
> > you have to close it before you open it again rule for files.  
> 
> This is an existing bug in this driver, I've fixed in the reflck series.
> 
> open_device/close_device will not run concurrently, or out of order,
> afer it is fixed.

Well if that is the case then provided your fix precedes this patch:

Acked-by: Halil Pasic <pasic@...ux.ibm.com>

I'm not entirely happy with this. I did not thoroughly investigate the
implications of reversing the locking order of the vfio-ap lock (driver
global) and the kvm lock (guest specific). I will trust Tony and hope
our KVM maintainers will scream if this is bad from interference and
delay perspective.

Regards,
Halil