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From:   Florian Weimer <fw@deneb.enyo.de>
To:     Chris Kennelly <ckennelly@google.com>
Cc:     Peter Oskolkov <posk@google.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Peter Oskolkov <posk@posk.io>,
        Peter Zijlstra <peterz@infradead.org>,
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        paulmck <paulmck@linux.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api <linux-api@vger.kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        carlos <carlos@redhat.com>
Subject: Re: [RFC PATCH 2/4] rseq: Allow extending struct rseq
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Date:   Wed, 15 Jul 2020 08:31:05 +0200
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        (Chris Kennelly's message of "Tue, 14 Jul 2020 22:34:38 -0400")
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* Chris Kennelly:

> When glibc provides registration, is the anticipated use case that a
> library would unregister and reregister each thread to "upgrade" it to
> the most modern version of interface it knows about provided by the
> kernel?

Absolutely not, that is likely to break other consumers because an
expected rseq area becomes dormant instead.

> There, I could assume an all-or-nothing registration of the new
> feature--limited only by kernel availability for thread
> homogeneity--but inconsistencies across early adopter libraries would
> mean each thread would have to examine its own TLS to determine if a
> feature were available.

Exactly.  Certain uses of seccomp can also have this effect,
presenting a non-homogeneous view.