Re: [RFC PATCH 2/4] rseq: Allow extending struct rseq

* 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.