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Re: Architecture baseline for Forky

Cross-posted to 2 groups.

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On Thu, Oct 30, 2025, at 09:47, Peter Green wrote:
> On 30/10/2025 08:28, Arnd Bergmann wrote:
>> On Thu, Oct 30, 2025, at 06:39, Peter Green wrote:
>>> The problem is that arm64 kernels by default trap the armv6k memory
>>> barriers into the kernel and emulate them. Asside from being slow
>>> this can also cause hangs if the optimiser moves a barrier inside
>>> a load-exclusive/store-exclusive loop.
>> Right, this is very unfortunate, and I suspect that this is something
>> we should change in both the arm64 kernel and in the compiler. If
>> you can point me to specific userspace code that has this problem,
>> I can probably come up with a kernel patch to work around it,
>> at least if that userspace code isn't obviously wrong.
> https://github.com/rust-lang/rust/issues/53670

Thanks for the pointer! This strongly suggests that the bug is
in llvm itself, not in userspace applications or the kernel.
I think the armv7-a version of the reproducer shows the same bug,
though that happens to work. This is probably one of the cases
where we want a compiler fix first, but can then add a workaround
in the host kernel (enabling the instruction itself instead of
the emulation where possible, same as the Raspberry Pi downstream
kernel does).

     Arnd

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Thread

Architecture baseline for Forky Bastian Blank <waldi@debian.org> - 2025-10-26 13:40 +0100
  Re: Architecture baseline for Forky John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de> - 2025-10-26 14:20 +0100
    Re: Architecture baseline for Forky Bastian Blank <waldi@debian.org> - 2025-10-27 08:40 +0100
  Re: Architecture baseline for Forky Tony Rodriguez <unixpro1970@gmail.com> - 2025-10-26 19:40 +0100
    Re: Architecture baseline for Forky John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de> - 2025-10-26 20:30 +0100
    Re: Architecture baseline for Forky Adrian Bunk <bunk@debian.org> - 2025-10-27 12:50 +0100
      Re: Architecture baseline for Forky Martin <debacle@debian.org> - 2025-10-27 18:10 +0100
      Re: Architecture baseline for Forky "Arnd Bergmann" <arnd@arndb.de> - 2025-10-27 22:40 +0100
        Re: Architecture baseline for Forky Adrian Bunk <bunk@debian.org> - 2025-10-29 09:00 +0100
          Re: Architecture baseline for Forky Peter Green <plugwash@debian.org> - 2025-10-30 07:00 +0100
            Re: Architecture baseline for Forky Peter Green <plugwash@debian.org> - 2025-10-30 10:10 +0100
              Re: Architecture baseline for Forky "Arnd Bergmann" <arnd@arndb.de> - 2025-10-30 10:10 +0100
          Re: Architecture baseline for Forky Adrian Bunk <bunk@debian.org> - 2025-10-30 15:30 +0100
    Re: Architecture baseline for Forky Rob Landley <rob@landley.net> - 2025-10-27 21:40 +0100
  Re: Architecture baseline for Forky Adrian Bunk <bunk@debian.org> - 2025-10-27 11:20 +0100
  Re: Architecture baseline for Forky Colin Watson <cjwatson@debian.org> - 2025-10-27 19:30 +0100
    Re: Architecture baseline for Forky "Nikos Tsipinakis" <nikos@tsipinakis.com> - 2025-10-30 14:40 +0100
  Re: Architecture baseline for Forky Milan Kupcevic <milan@debian.org> - 2025-10-27 22:40 +0100
    Re: Architecture baseline for Forky Bastian Blank <waldi@debian.org> - 2025-10-27 23:40 +0100
      Re: Architecture baseline for Forky Marc Haber <mh+debian-release@zugschlus.de> - 2025-10-28 10:10 +0100
    Planned obsolescence ? (was: Re: Architecture baseline for Forky) Romain Dolbeau <romain@dolbeau.org> - 2025-10-28 10:00 +0100
      Re: Planned obsolescence ? (OpenBSD) Paul Tagliamonte <paultag@debian.org> - 2025-11-02 23:30 +0100
      Re: Planned obsolescence ? (*BSD, Rust) David Starner <prosfilaes@gmail.com> - 2025-11-04 00:20 +0100
  Re: Architecture baseline for Forky Rob Landley <rob@landley.net> - 2025-12-05 00:20 +0100
  R: Re: Architecture baseline for Forky Federico Vaga <federico.vaga@vaga.pv.it> - 2026-09-07 09:20 +0200

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