[PATCH v4 1/4] arm64: fpsimd: Drop unneeded 'busy' flag

[Geert Uytterhoeven]

Hi Ard,

On Fri, Dec 8, 2023 at 12:34 PM Ard Biesheuvel <ardb at google.com> wrote:
> From: Ard Biesheuvel <ardb at kernel.org>
> Kernel mode NEON will preserve the user mode FPSIMD state by saving it
> into the task struct before clobbering the registers. In order to avoid
> the need for preserving kernel mode state too, we disallow nested use of
> kernel mode NEON, i..e, use in softirq context while the interrupted
> task context was using kernel mode NEON too.
>
> Originally, this policy was implemented using a per-CPU flag which was
> exposed via may_use_simd(), requiring the users of the kernel mode NEON
> to deal with the possibility that it might return false, and having NEON
> and non-NEON code paths. This policy was changed by commit
> 13150149aa6ded1 ("arm64: fpsimd: run kernel mode NEON with softirqs
> disabled"), and now, softirq processing is disabled entirely instead,
> and so may_use_simd() can never fail when called from task or softirq
> context.
>
> This means we can drop the fpsimd_context_busy flag entirely, and
> instead, ensure that we disable softirq processing in places where we
> formerly relied on the flag for preventing races in the FPSIMD preserve
> routines.
>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> Reviewed-by: Mark Brown <broonie at kernel.org>

Thanks for your patch, which is now commit e109130b0e5ec3fd
("arm64: fpsimd: Drop unneeded 'busy' flag") in arm64/for-next/core
and next-20231212.

I have bisected the following warning during boot (on Salvator-XS with
R-Car H3 ES2.0 and on White-Hawk with R-Car V4H) followed by a lock-up
(on Salvator-XS) to this commit:

Reverting commits 035262623959cbe1 ("arm64: fpsimd: Implement lazy
restore for kernel mode FPSIMD"), 1e3a3de1ff6ca6b1 ("arm64: fpsimd:
Preserve/restore kernel mode NEON at context switch"), and this commit
fixes the issue.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds