Re: [RFC PATCH 1/2] glibc: Perform rseq(2) registration at nptl init and thread creation (v3)

[Andy Lutomirski]

On Fri, Nov 2, 2018 at 4:53 AM Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>
> Here is a third round of prototype registering rseq(2) TLS for each
> thread (including main), and unregistering for each thread (excluding
> main). "rseq" stands for Restartable Sequences.
>
> Remaining open questions:
>
> - How early do we want to register rseq and how late do we want to
>   unregister it ? It's important to consider if we expect rseq to
>   be used by the memory allocator and within destructor callbacks.
>   However, we want to be sure the TLS (__thread) area is properly
>   allocated across its entire use by rseq.
>
> - We do not need an atomic increment/decrement for the refcount per
>   se. Just being atomic with respect to the current thread (and nested
>   signals) would be enough. What is the proper API to use there ?
>
> See the rseq(2) man page proposed here:
>   https://lkml.org/lkml/2018/9/19/647
>

Merely having rseq registered carries some small but nonzero overhead,
right?  Should this perhaps live in a librseq.so or similar (possibly
built as part of libc) to avoid the overhead for programs that don't
use it?