linux-kernel - Re: [PATCH v2 1/9] s390/uaccess: Add storage key checked cmpxchg access to user space

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Date:   Thu, 17 Nov 2022 09:42:43 +0100
From:   Nico Boehr <nrb@...ux.ibm.com>
To:     Heiko Carstens <hca@...ux.ibm.com>,
        Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
Cc:     Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Jonathan Corbet <corbet@....net>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-s390@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Sven Schnelle <svens@...ux.ibm.com>
Subject: Re: [PATCH v2 1/9] s390/uaccess: Add storage key checked cmpxchg access to user space

Quoting Janis Schoetterl-Glausch (2022-11-16 20:36:46)
> On Wed, 2022-11-02 at 15:12 +0100, Heiko Carstens wrote:
> > 
> [...]
> 
> > I also did not limit the number of retries for the one and two byte
> > scenarion. Before doing that we need to have proof that there really is a
> > problem. Maybe Nico or you will give this a try.
> 
> I wrote a memop selftest testcase where the main thread uses the one byte cmpxchg
> while n vcpus flip adjacent bits. The time the test case runs increases superlinearly with n.
> With 248 vcpus, 1000 one byte cmpxchgs take 25s.
> I'm not sure how meaningful the test is since the worst case would be if the threads hammering
> the word would run on a cpu dedicated to them.
> 
> In any case, why not err on the side of caution and limit the iterations?
> I'll send an rfc patch.

I agree, limiting sounds like the safe choice.