Re: [PATCH 2/3] Linux: Use rseq in sched_getcpu if available (v9)

[Florian Weimer <fweimer@redhat.com>]

* Mathieu Desnoyers:

> Now we need to discuss how we introduce that fix in a way that will
> allow user-space to trust the __rseq_abi.cpu_id field's content.

I don't think that's necessary.  We can mention it in the glibc
distribution notes on the wiki.

> The usual approach to kernel bug fixing is typically to push the fix,
> mark it for stable kernels, and expect everyone to pick up the
> fixes. I wonder how comfortable glibc would be to replace its
> sched_getcpu implementation with a broken-until-fixed kernel rseq
> implementation without any mechanism in place to know whether it can
> trust the value of the cpu_id field. I am extremely reluctant to do
> so.

We have already had similar regressions in sched_getcpu, and we didn't
put anything into glibc to deal with those.

Just queue the fix for the stable kernels.  I expect that all
distributions track stable kernel branches in some way, so just put into
the kernel commit message that this commit is needed for a working
sched_getcpu in glibc 2.32 and later.

Once the upstream fix is in Linus' tree, I'm going to file a request to
backport the fix into the Red Hat Enterprise Linux 8.

Thanks for finding the root cause so quickly,
Florian