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Re: [RFC PATCH] glibc: Perform rseq(2) registration at nptl init and thread creation
- From: Mathieu Desnoyers <mathieu dot desnoyers at efficios dot com>
- To: Joseph Myers <joseph at codesourcery dot com>
- Cc: Szabolcs Nagy <szabolcs dot nagy at arm dot com>, carlos <carlos at redhat dot com>, Florian Weimer <fweimer at redhat dot com>, nd <nd at arm dot com>, Thomas Gleixner <tglx at linutronix dot de>, Ben Maurer <bmaurer at fb dot com>, Peter Zijlstra <peterz at infradead dot org>, "Paul E. McKenney" <paulmck at linux dot vnet dot ibm dot com>, Boqun Feng <boqun dot feng at gmail dot com>, Will Deacon <will dot deacon at arm dot com>, Dave Watson <davejwatson at fb dot com>, Paul Turner <pjt at google dot com>, libc-alpha <libc-alpha at sourceware dot org>, linux-kernel <linux-kernel at vger dot kernel dot org>, linux-api <linux-api at vger dot kernel dot org>
- Date: Wed, 19 Sep 2018 16:10:54 -0400 (EDT)
- Subject: Re: [RFC PATCH] glibc: Perform rseq(2) registration at nptl init and thread creation
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----- On Sep 19, 2018, at 3:49 PM, Joseph Myers firstname.lastname@example.org wrote:
> On Wed, 19 Sep 2018, Szabolcs Nagy wrote:
>> i don't think there is precedent for exposing tls symbol in glibc
>> (e.g. errno is exposed via __errno_location function) so there
>> might be issues with this (but i don't have immediate concerns).
> There have been suggestions to expose TLS errno - but also suggestions
> that use of __errno_location is more efficient, at least in terms of code
> size everywhere errno is accessed (for some ABIs, anyway).
AFAIU, the trade-off is different between the errno use-case and the
If my understanding is correct, errno is not supposed to be used in
fast-paths, only when an error is returned. So size is more important than
Comparatively, rseq is _meant_ to speed up fast-paths. A per-cpu statistics
counter can be incremented in 2ns with rseq on a Intel E5-2630, which is faster
than a simple function call.
So for rseq, we should favor speed over space, which means the user applications
and libraries would need access to the TLS symbol without requiring an
accessor function. That's also why I'm using the initial-exec tls model
rather than the global-dynamic: I want to make sure no function call is
> The ABI tests have code that would list .tbss symbols as "T" in ABI test
> baselines, but no existing ABI baselines use that.