From mboxrd@z Thu Jan  1 00:00:00 1970
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Re: [RFC PATCH glibc 1/4] glibc: Perform rseq(2) registration at
 nptl init and thread creation (v4)
Date: Mon, 14 Jan 2019 14:22:37 -0500 (EST)
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To: Florian Weimer <fweimer@redhat.com>
Cc: carlos <carlos@redhat.com>, Joseph Myers <joseph@codesourcery.com>, Szabolcs Nagy <szabolcs.nagy@arm.com>, libc-alpha <libc-alpha@sourceware.org>, 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>, Rich Felker <dalias@libc.org>, linux-kernel <linux-kernel@vger.kernel.org>, linux-api <linux-api@vger.kernel.org>
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----- On Jan 14, 2019, at 10:55 AM, Florian Weimer fweimer@redhat.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