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Date:   Wed, 2 Feb 2022 08:08:55 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Florian Weimer <fweimer@redhat.com>
Cc:     Chris Kennelly <ckennelly@google.com>,
        Paul Turner <pjt@google.com>, Peter Oskolkov <posk@posk.io>,
        libc-alpha <libc-alpha@sourceware.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
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Subject: Re: Aligning tcmalloc with glibc 2.35 rseq ABI
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Thread-Topic: Aligning tcmalloc with glibc 2.35 rseq ABI
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----- On Feb 2, 2022, at 6:36 AM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:

> ----- On Feb 2, 2022, at 3:41 AM, Florian Weimer fweimer@redhat.com wrote:
> 
>> * Florian Weimer:
>> 
>>> * 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.
>> 
>> And now I'm kind of worried that we should be using ptrdiff_t for
>> __rseq_offset because that's what the initial-exec relocations use. 8-/
> 
> I suspect the underlying question here is: how likely is it that a libc
> requires an offset of more than 2GB either way from the thread pointer
> to allocate its rseq thread area on a 64-bit architecture ?

More to the point: is ptrdiff_t the correct type here ? I think so.
Do we want to revert the ABI and wait another 6 months before we
bring back rseq into glibc just for this ? I'm not sure this limitation
justifies it.

So if there is a quick way to fix that before the official 2.35 release,
I'm all for it, otherwise I cannot say that __rseq_offset being an "int"
rather than a "ptrdiff_t" will make much real-life difference (unless
I'm proven wrong). But we will be stuck with this quirk forever.

Thanks,

Mathieu


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