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

[Will Deacon]

Hi Geert,

Cheers for the report.

On Tue, Dec 12, 2023 at 12:27:50PM +0100, Geert Uytterhoeven wrote:
> 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:

Please can you provide the output from the warning and, if possible, a
pointer to your .config?

Cheers,

Will