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From: Florian Weimer <fweimer@redhat.com>
To: Chris Kennelly <ckennelly@google.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,  Paul Turner
 <pjt@google.com>,  Peter Oskolkov <posk@posk.io>,  libc-alpha@sourceware.org
 <libc-alpha@sourceware.org>,  linux-kernel <linux-kernel@vger.kernel.org>,
 Peter Zijlstra <peterz@infradead.org>
Subject: Re: Aligning tcmalloc with glibc 2.35 rseq ABI
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 <CAEE+ybn_Cp1-T=2uB7xJqU2gEU-PxzsaV5jqCOupNp2cx_bK-Q@mail.gmail.com>
Date: Tue, 01 Feb 2022 21:39:16 +0100
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 (Chris Kennelly's message of "Tue, 1 Feb 2022 15:33:15 -0500")
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* Chris Kennelly:

> Thanks for the heads up.
>
> I did have a question about whether the new protocol would introduce
> an extra memory reference while initializing a critical section.
>
> * With initial-exec TLS, I can directly reference __rseq_abi.
> * With the new ABI, I might need to ask glibc for the address of the
> registered rseq structure in its thread data.

You can write __rseq_offset to a static/hidden variable in an ELF
constructor, and then use pretty much the same assembler sequences as
for initial-exec TLS on most architectures.

Thanks,
Florian