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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Fri, 2 Nov 2018 08:20:18 -0700
Message-ID: <CALCETrWG4j7R7jzuoPW6ifYVzwuQGMEHBp7Pb_k66_2hn1wUHg@mail.gmail.com>
Subject: Re: [RFC PATCH 1/2] glibc: Perform rseq(2) registration at nptl init
 and thread creation (v3)
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     libc-alpha <libc-alpha@sourceware.org>,
        "Carlos O'Donell" <carlos@redhat.com>,
        Florian Weimer <fweimer@redhat.com>,
        "Joseph S. Myers" <joseph@codesourcery.com>, szabolcs.nagy@arm.com,
        Thomas Gleixner <tglx@linutronix.de>,
        Ben Maurer <bmaurer@fb.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Will Deacon <will.deacon@arm.com>,
        Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>
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On Fri, Nov 2, 2018 at 4:53 AM Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> 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?