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From:   Florian Weimer <fweimer@redhat.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     carlos <carlos@redhat.com>, Joseph Myers <joseph@codesourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@arm.com>,
        libc-alpha <libc-alpha@sourceware.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ben Maurer <bmaurer@fb.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Paul <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Will Deacon <will.deacon@arm.com>,
        Paul Turner <pjt@google.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>
Subject: Re: [PATCH 2/3] Linux: Use rseq in sched_getcpu if available (v9)
References: <20200629190036.26982-1-mathieu.desnoyers@efficios.com>
        <20200629190036.26982-3-mathieu.desnoyers@efficios.com>
        <877dvg4ud4.fsf@oldenburg2.str.redhat.com>
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        <1679448037.22891.1594056826859.JavaMail.zimbra@efficios.com>
Date:   Mon, 06 Jul 2020 19:50:54 +0200
In-Reply-To: <1679448037.22891.1594056826859.JavaMail.zimbra@efficios.com>
        (Mathieu Desnoyers's message of "Mon, 6 Jul 2020 13:33:46 -0400
        (EDT)")
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* 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