Re: [PATCH v2 1/2] rwonce: Add READ_ONCE_THIS_CPU() and WRITE_ONCE_THIS_CPU()

[Paul E. McKenney]

On Fri, Oct 02, 2026 at 11:17:55PM +0800, Kunwu Chan wrote:
> Per-CPU variables are sometimes accessed from different execution
> contexts on the same CPU, typically task context and an interrupt or
> NMI handler.
> 
> A plain READ_ONCE() or WRITE_ONCE() marks such an access as an
> intentional race but does not express where the racing accesses are
> expected to come from. This distinction allows concurrency tools such
> as KCSAN to tell an intentional same-CPU context race apart from an
> access that may race across CPUs.
> 
> Add READ_ONCE_THIS_CPU() and WRITE_ONCE_THIS_CPU() to identify accesses
> to a per-CPU variable which are intended to race only with accesses
> from other execution contexts on the same CPU. For now, they are
> simple wrappers around READ_ONCE() and WRITE_ONCE(), so generated code
> and tool behaviour are unchanged.
> 
> Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxx>
> Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Signed-off-by: Kunwu Chan <kunwu.chan@xxxxxxxxx>

Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> ---
>  include/asm-generic/rwonce.h | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/include/asm-generic/rwonce.h b/include/asm-generic/rwonce.h
> index 52b969c7cef9..4d4e009a4489 100644
> --- a/include/asm-generic/rwonce.h
> +++ b/include/asm-generic/rwonce.h
> @@ -61,6 +61,17 @@ do {									\
>  	__WRITE_ONCE(x, val);						\
>  } while (0)
>  
> +/*
> + * {READ,WRITE}_ONCE_THIS_CPU() identify accesses to a per-CPU variable
> + * which are intended to race only with accesses from other execution
> + * contexts on the same CPU, e.g. task and interrupt context.
> + *
> + * For now, these are simple wrappers around READ_ONCE() and
> + * WRITE_ONCE().
> + */
> +#define READ_ONCE_THIS_CPU(x)		READ_ONCE(x)
> +#define WRITE_ONCE_THIS_CPU(x, val)	WRITE_ONCE(x, val)
> +
>  static __no_sanitize_or_inline
>  unsigned long __read_once_word_nocheck(const void *addr)
>  {
> -- 
> 2.43.0
>