linux-kernel - Re: [RFC PATCH glibc 1/4] glibc: Perform rseq(2) registration at nptl init and thread creation (v4)

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Date:   Mon, 14 Jan 2019 14:22:37 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     carlos <carlos@...hat.com>, Joseph Myers <joseph@...esourcery.com>,
        Szabolcs Nagy <szabolcs.nagy@....com>,
        libc-alpha <libc-alpha@...rceware.org>,
        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>,
        Rich Felker <dalias@...c.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>
Subject: Re: [RFC PATCH glibc 1/4] glibc: Perform rseq(2) registration at
 nptl init and thread creation (v4)

----- On Jan 14, 2019, at 10:55 AM, Florian Weimer fweimer@...hat.com wrote:

> * Mathieu Desnoyers:
> 
>> Therefore, both symbols will end up in
>> sysdeps/unix/sysv/linux/Versions.
> 
> I'm not sure what you mean by that.  The physical location in the
> directory tree has little effect on which shared object the symbol is
> placed in; that will need other changes.

I'm currently moving the symbol definitions to csu/rseq-sym.c. On Linux,
its content is overridden by a new sysdeps/unix/sysv/linux/rseq-sym.c
which contains both __rseq_abi and __rseq_refcount symbols. On other
platforms, it is a stub file.

>>> By the way, you could avoid the need for unregistration if you allocated
>>> the rseq areas persistently, index by TID.  They are quite small, so
>>> with the typical PID range, maybe the wasted memory due to changing TIDs
>>> would be acceptable?
>>
>> Would we be able to access those __rseq_abi as normal TLS IE model
>> variables ?  The overhead of indexing an array matters for a
>> fast-path.
> 
> No, that wouldn't be possible in this case.  You would need another
> indirection.

Thanks for the clarification!

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com