linux-kernel - Re: [PATCH v17 14/15] s390/ap: notify drivers on config changed and scan complete callbacks

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Date:   Tue, 8 Feb 2022 02:38:35 +0100
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,
        kvm@...r.kernel.org, jjherne@...ux.ibm.com, freude@...ux.ibm.com,
        borntraeger@...ibm.com, cohuck@...hat.com, mjrosato@...ux.ibm.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        fiuczy@...ux.ibm.com, Halil Pasic <pasic@...ux.ibm.com>
Subject: Re: [PATCH v17 14/15] s390/ap: notify drivers on config changed and
 scan complete callbacks

On Mon, 7 Feb 2022 14:39:31 -0500
Tony Krowiak <akrowiak@...ux.ibm.com> wrote:

> > Back to the topic of locking: it looks to me that on this path you
> > do the filtering and thus the accesses to matrix_mdev->shadow_apcb,
> > matrix_mdev->matrix and matrix_dev->config_info some of which are
> > of type write whithout the matrix_dev->lock held. More precisely
> > only with the matrix_dev->guests_lock held in "read" mode.
> >
> > Did I misread the code? If not, how is that OK?  
> 
> You make a valid point, a struct rw_semaphore is not adequate for the 
> purposes
> it is used in this patch series. It needs to be a mutex.
> 

Good we agree that v17 is racy.

> 
> For v18 which is forthcoming probably this week, I've been reworking the 
> locking
> based on your observation that the struct ap_guest is not necessary given we
> already have a list of the mediated devices which contain the KVM 
> pointer. On the other

[..]
> 
> >
> > BTW I got delayed on my "locking rules" writeup. Sorry for that!  
> 
> No worries, I've been writing up a vfio-ap-locking.rst document to 
> include with the next
> version of the patch series.

I'm looking forward to v18 including that document. I prefer not to
discuss what you wrote about the approach taken in v18 now. It is easier
to me when I have both the text stating the intended design, and the
code that is supposed to adhere to this design.

Regards,
Halil