Re: [PATCH] csky: cmpxchg: Optimize with acquire & release

Mark Rutland
2022-04-11T09:38:01Z

Hi Guo,

On Wed, Apr 06, 2022 at 08:40:56PM +0800, guoren@kernel.org wrote:
> From: Guo Ren <guoren@linux.alibaba.com>
>
> Optimize arch_xchg|cmpxchg|cmpxchg_local with ASM acquire|release
> instructions instead of previous C based.
>
> Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> Signed-off-by: Guo Ren <guoren@kernel.org>
> ---
> arch/csky/include/asm/barrier.h | 8 +-
> arch/csky/include/asm/cmpxchg.h | 173 ++++++++++++++++++++++++++++++--
> 2 files changed, 172 insertions(+), 9 deletions(-)
>
> diff --git a/arch/csky/include/asm/barrier.h b/arch/csky/include/asm/barrier.h
> index f4045dd53e17..a075f17d02dd 100644
> --- a/arch/csky/include/asm/barrier.h
> +++ b/arch/csky/include/asm/barrier.h
> @@ -37,6 +37,9 @@
> * bar.brar
> * bar.bwaw
> */
> +#define ACQUIRE_FENCE ".long 0x8427c000\n"
> +#define RELEASE_FENCE ".long 0x842ec000\n"

[...]

> +#define __xchg(new, ptr, size) \
> +({ \
> + __typeof__(ptr) __ptr = (ptr); \
> + __typeof__(new) __new = (new); \
> + __typeof__(*(ptr)) __ret; \
> + unsigned long tmp; \
> + switch (size) { \
> + case 4: \
> + asm volatile ( \
> + "1: ldex.w %0, (%3) \n" \
> + ACQUIRE_FENCE \
> + " mov %1, %2 \n" \
> + RELEASE_FENCE \
> + " stex.w %1, (%3) \n" \
> + " bez %1, 1b \n" \
> + : "=&r" (__ret), "=&r" (tmp) \
> + : "r" (__new), "r"(__ptr) \
> + :); \

As with the riscv case, I believe this suffers the problem described in:

8e86f0b409a44193 ("arm64: atomics: fix use of acquire + release for full barrier semantics")

... and does not provide FULL ordering semantics.

[...]

> +#define __cmpxchg(ptr, old, new, size) \
> +({ \
> + __typeof__(ptr) __ptr = (ptr); \
> + __typeof__(new) __new = (new); \
> + __typeof__(new) __tmp; \
> + __typeof__(old) __old = (old); \
> + __typeof__(*(ptr)) __ret; \
> + switch (size) { \
> + case 4: \
> + asm volatile ( \
> + "1: ldex.w %0, (%3) \n" \
> + ACQUIRE_FENCE \
> + " cmpne %0, %4 \n" \
> + " bt 2f \n" \
> + " mov %1, %2 \n" \
> + RELEASE_FENCE \
> + " stex.w %1, (%3) \n" \
> + " bez %1, 1b \n" \
> + "2: \n" \
> + : "=&r" (__ret), "=&r" (__tmp) \
> + : "r" (__new), "r"(__ptr), "r"(__old) \
> + :); \

Likewise.

Thanks,
Mark.