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

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Date:   Mon, 7 Feb 2022 22:27:03 -0500
From:   Tony Krowiak <akrowiak@...ux.ibm.com>
To:     Halil Pasic <pasic@...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
Subject: Re: [PATCH v17 14/15] s390/ap: notify drivers on config changed and
 scan complete callbacks



On 2/7/22 20:38, Halil Pasic wrote:
> 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

Coming soon to a theater near you:)

>