On Tue, Jul 10, 2018 at 10:21:40AM +0800, Yandong.Zhao wrote:
> From: Yandong Zhao <yandong77...@gmail.com>
>
> Operations for contexts where we do not want to do any checks for
> preemptions. Unless strictly necessary, always use this_cpu_read()
> instead. Because of the kernel_neon_busy here we have to make sure
> that it is the current cpu.
I find this wording a bit confusing.
Does the following make look OK to you?
--8<--
It does not matter if the caller of may_use_simd() migrates to
another cpu after the call, but it is still important that the
kernel_neon_busy percpu instance that is read matches the cpu the
task is running on at the time of the read.
This means that raw_cpu_read() is not sufficient. kernel_neon_busy
may appear true if the caller migrates during the execution of
raw_cpu_read() and the next task to be scheduled in on the initial
cpu calls kernel_neon_begin().
This patch replaces raw_cpu_read() with this_cpu_read() to protect
against this race.
-->8--
>
> Signed-off-by: Yandong Zhao <yandong77...@gmail.com>
> ---
> arch/arm64/include/asm/simd.h | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/include/asm/simd.h b/arch/arm64/include/asm/simd.h
> index fa8b3fe..8b97f8b 100644
> --- a/arch/arm64/include/asm/simd.h
> +++ b/arch/arm64/include/asm/simd.h
> @@ -29,7 +29,8 @@
> static __must_check inline bool may_use_simd(void)
> {
> /*
> - * The raw_cpu_read() is racy if called with preemption enabled.
> + * The this_cpu_read() is racy if called with preemption enabled,
> + * since the task my subsequently migrate to another CPU.
"my" -> "may"
(apologies if I was responsible for that typo)
[...]
Cheers
---Dave
