Re: [PATCH RFC] arm64: Mark set_preempt_need_resched() access to .need_resched

[Mark Rutland <mark.rutland@arm.com>]

Hi Paul,

On Thu, Jul 30, 2026 at 04:59:57PM -0700, Paul E. McKenney wrote:
> The .need_resched field can be accessed from both task level and
> from interrrupt handlers, so apply WRITE_ONCE() to the update in
> set_preempt_need_resched().  This also brings arm64 in line with s390
> (which uses atomic operations) and x86 (which uses inline assembly).
> Other architectures avoid this issue via the empty definition in
> include/asm-generic/preempt.h.
> 
> KCSAN located this issue.

Do you have the KCSAN splat to hand? Seeing the exact race (and the
relevant reader(s)) would be handy.

> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Jinjie Ruan <ruanjinjie@huawei.com>
> Cc: Ada Couprie Diaz <ada.coupriediaz@arm.com>
> Cc: <linux-arm-kernel@lists.infradead.org>
> 
> ---
>  preempt.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/preempt.h b/arch/arm64/include/asm/preempt.h
> index 932ea4b6204289..610853da140a78 100644
> --- a/arch/arm64/include/asm/preempt.h
> +++ b/arch/arm64/include/asm/preempt.h
> @@ -28,7 +28,7 @@ static inline void preempt_count_set(u64 pc)
>  
>  static inline void set_preempt_need_resched(void)
>  {
> -	current_thread_info()->preempt.need_resched = 0;
> +	WRITE_ONCE(current_thread_info()->preempt.need_resched, 0);
>  }

I was under the impression that we only used this in a context where we
couldn't have a nested writer (e.g. within a scheduler IRQ hook, with
IRQs disabled).

If we have concurrent writers, this is probably broken regardless of
whether it is single-copy-atomic, and I worry WRITE_ONCE() will mask a
bug.

If you can share the KCSAN splat, that'd really help.

I think we need READ_ONCE() in test_preempt_need_resched(), but today
that's only used by tracing to determine what to log.

Mark.