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

[Paul E. McKenney]

On Fri, Jul 31, 2026 at 01:51:26PM +0100, Mark Rutland wrote:
> 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 at kernel.org>
> > Cc: Catalin Marinas <catalin.marinas at arm.com>
> > Cc: Will Deacon <will at kernel.org>
> > Cc: Jinjie Ruan <ruanjinjie at huawei.com>
> > Cc: Ada Couprie Diaz <ada.coupriediaz at arm.com>
> > Cc: <linux-arm-kernel at 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.

Let me regenerate it...

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

I do have tracing enabled on some of my runs.

							Thanx, Paul