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

Nico Boehr
2022-11-17T08:43:46Z

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.