linux-kernel - Re: [RFC PATCH] glibc: Perform rseq(2) registration at nptl init and thread creation

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Message-ID: <alpine.DEB.2.21.1809202016270.17228@digraph.polyomino.org.uk>
Date:   Thu, 20 Sep 2018 20:20:28 +0000
From:   Joseph Myers <joseph@...esourcery.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC:     carlos <carlos@...hat.com>, Florian Weimer <fweimer@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ben Maurer <bmaurer@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will.deacon@....com>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        libc-alpha <libc-alpha@...rceware.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>
Subject: Re: [RFC PATCH] glibc: Perform rseq(2) registration at nptl init
 and thread creation

On Thu, 20 Sep 2018, Mathieu Desnoyers wrote:

> Are you saying glibc has an explicit check for the kernel version visible
> from /proc before using specific features ? If so, how can this work with
> the variety of feature backports we find in the distribution kernels out
> there ?

See sysdeps/unix/sysv/linux/dl-sysdep.c and 
sysdeps/unix/sysv/linux/dl-osinfo.h.  As I said, Carlos has proposed 
removing that check.

> For too-old headers at compile time, one possibility is that we don't event
> expose the __rseq_abi TLS symbol. OTOH, if we need to keep exposing it anyway
> for ABI consistency purposes, then we'd leave its cpu_id field at the initial
> value (-1). But that would require that we copy linux/rseq.h into the glibc
> source tree.

The ABI needs to be independent of the kernel headers used.  I don't think 
you need to copy linux/rseq.h; all you should need is to e.g. define an 
array of suitable size and alignment with the relevant member initialized 
and a suitable explanatory comment.

-- 
Joseph S. Myers
joseph@...esourcery.com