Re: [PATCH v3 1/5] arch,locking/atomic: arc: arch_cmpxchg should check data size

[Andi Shyti]

Hi Wuqiang,

On Tue, Nov 21, 2023 at 10:23:43PM +0800, wuqiang.matt wrote:
> arch_cmpxchg() should check data size rather than pointer size in case
> CONFIG_ARC_HAS_LLSC is defined. So rename __cmpxchg to __cmpxchg_32 to
> emphasize it's explicit support of 32bit data size with BUILD_BUG_ON()
> added to avoid any possible misuses with unsupported data types.
> 
> In case CONFIG_ARC_HAS_LLSC is undefined, arch_cmpxchg() uses spinlock
> to accomplish SMP-safety, so the BUILD_BUG_ON checking is uncecessary.
> 
> v2 -> v3:
>   - Patches regrouped and has the improvement for xtensa included
>   - Comments refined to address why these changes are needed
> 
> v1 -> v2:
>   - Try using native cmpxchg variants if avaialble, as Arnd advised
> 
> Signed-off-by: wuqiang.matt <wuqiang.matt@xxxxxxxxxxxxx>
> Reviewed-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> ---
>  arch/arc/include/asm/cmpxchg.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arc/include/asm/cmpxchg.h b/arch/arc/include/asm/cmpxchg.h
> index e138fde067de..bf46514f6f12 100644
> --- a/arch/arc/include/asm/cmpxchg.h
> +++ b/arch/arc/include/asm/cmpxchg.h
> @@ -18,14 +18,16 @@
>   * if (*ptr == @old)
>   *      *ptr = @new
>   */
> -#define __cmpxchg(ptr, old, new)					\
> +#define __cmpxchg_32(ptr, old, new)					\
>  ({									\
>  	__typeof__(*(ptr)) _prev;					\
>  									\
> +	BUILD_BUG_ON(sizeof(*(ptr)) != 4);				\
> +									\
>  	__asm__ __volatile__(						\
> -	"1:	llock  %0, [%1]	\n"					\
> +	"1:	llock  %0, [%1]		\n"				\
>  	"	brne   %0, %2, 2f	\n"				\
> -	"	scond  %3, [%1]	\n"					\
> +	"	scond  %3, [%1]		\n"				\
>  	"	bnz     1b		\n"				\
>  	"2:				\n"				\
>  	: "=&r"(_prev)	/* Early clobber prevent reg reuse */		\
> @@ -47,7 +49,7 @@
>  									\
>  	switch(sizeof((_p_))) {						\
>  	case 4:								\
> -		_prev_ = __cmpxchg(_p_, _o_, _n_);			\
> +		_prev_ = __cmpxchg_32(_p_, _o_, _n_);			\
>  		break;							\
>  	default:							\
>  		BUILD_BUG();						\
> @@ -65,8 +67,6 @@
>  	__typeof__(*(ptr)) _prev_;					\
>  	unsigned long __flags;						\
>  									\
> -	BUILD_BUG_ON(sizeof(_p_) != 4);					\
> -									\

I think I made some comments here that have not been addressed or
replied.

Thanks,
Andi

>  	/*								\
>  	 * spin lock/unlock provide the needed smp_mb() before/after	\
>  	 */								\
> -- 
> 2.40.1