Re: [PATCH v2 07/10] riscv: Improve amoswap.X use in xchg()

Andrea Parri
2024-06-27T13:58:42Z

On Wed, Jun 26, 2024 at 03:03:44PM +0200, Alexandre Ghiti wrote:
> xchg() uses amoswap.X instructions from Zabha but still uses
> the LR/SC acquire/release semantics which require barriers.
>
> Let's improve that by using proper amoswap acquire/release semantics in
> order to avoid any of those barriers.
>
> Suggested-by: Andrea Parri <andrea@rivosinc.com>
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> ---
> arch/riscv/include/asm/cmpxchg.h | 35 +++++++++++++-------------------
> 1 file changed, 14 insertions(+), 21 deletions(-)
>
> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> index eb35e2d30a97..0e57d5fbf227 100644
> --- a/arch/riscv/include/asm/cmpxchg.h
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -11,8 +11,8 @@
> #include <asm/fence.h>
> #include <asm/alternative.h>
>
> -#define __arch_xchg_masked(sc_sfx, swap_sfx, prepend, sc_append, \
> - swap_append, r, p, n) \
> +#define __arch_xchg_masked(sc_sfx, swap_sfx, sc_prepend, sc_append, \
> + r, p, n) \
> ({ \
> __label__ zabha, end; \
> \
> @@ -31,7 +31,7 @@
> ulong __rc; \
> \
> __asm__ __volatile__ ( \
> - prepend \
> + sc_prepend \
> "0: lr.w %0, %2\n" \
> " and %1, %0, %z4\n" \
> " or %1, %1, %z3\n" \
> @@ -48,9 +48,7 @@
> zabha: \
> if (IS_ENABLED(CONFIG_RISCV_ISA_ZABHA)) { \
> __asm__ __volatile__ ( \
> - prepend \
> " amoswap" swap_sfx " %0, %z2, %1\n" \
> - swap_append \
> : "=&r" (r), "+A" (*(p)) \
> : "rJ" (n) \
> : "memory"); \
> @@ -58,19 +56,17 @@ zabha: \
> end:; \
> })
>
> -#define __arch_xchg(sfx, prepend, append, r, p, n) \
> +#define __arch_xchg(sfx, r, p, n) \
> ({ \
> __asm__ __volatile__ ( \
> - prepend \
> " amoswap" sfx " %0, %2, %1\n" \
> - append \
> : "=r" (r), "+A" (*(p)) \
> : "r" (n) \
> : "memory"); \
> })
>
> -#define _arch_xchg(ptr, new, sc_sfx, swap_sfx, prepend, \
> - sc_append, swap_append) \
> +#define _arch_xchg(ptr, new, sc_sfx, swap_sfx, \
> + sc_prepend, sc_append) \
> ({ \
> __typeof__(ptr) __ptr = (ptr); \
> __typeof__(*(__ptr)) __new = (new); \
> @@ -79,21 +75,19 @@ end:; \
> switch (sizeof(*__ptr)) { \
> case 1: \
> __arch_xchg_masked(sc_sfx, ".b" swap_sfx, \
> - prepend, sc_append, swap_append, \
> + sc_prepend, sc_append, \
> __ret, __ptr, __new); \
> break; \
> case 2: \
> __arch_xchg_masked(sc_sfx, ".h" swap_sfx, \
> - prepend, sc_append, swap_append, \
> + sc_prepend, sc_append, \
> __ret, __ptr, __new); \
> break; \
> case 4: \
> - __arch_xchg(".w" swap_sfx, prepend, swap_append, \
> - __ret, __ptr, __new); \
> + __arch_xchg(".w" swap_sfx, __ret, __ptr, __new); \
> break; \
> case 8: \
> - __arch_xchg(".d" swap_sfx, prepend, swap_append, \
> - __ret, __ptr, __new); \
> + __arch_xchg(".d" swap_sfx, __ret, __ptr, __new); \
> break; \
> default: \
> BUILD_BUG(); \
> @@ -102,17 +96,16 @@ end:; \
> })
>
> #define arch_xchg_relaxed(ptr, x) \
> - _arch_xchg(ptr, x, "", "", "", "", "")
> + _arch_xchg(ptr, x, "", "", "", "")
>
> #define arch_xchg_acquire(ptr, x) \
> - _arch_xchg(ptr, x, "", "", "", \
> - RISCV_ACQUIRE_BARRIER, RISCV_ACQUIRE_BARRIER)
> + _arch_xchg(ptr, x, "", ".aq", "", RISCV_ACQUIRE_BARRIER)
>
> #define arch_xchg_release(ptr, x) \
> - _arch_xchg(ptr, x, "", "", RISCV_RELEASE_BARRIER, "", "")
> + _arch_xchg(ptr, x, "", ".rl", RISCV_RELEASE_BARRIER, "")
>
> #define arch_xchg(ptr, x) \
> - _arch_xchg(ptr, x, ".rl", ".aqrl", "", RISCV_FULL_BARRIER, "")
> + _arch_xchg(ptr, x, ".rl", ".aqrl", "", RISCV_FULL_BARRIER)

I actually see no reason for this patch, please see also my remarks
/question on patch #4.

Andrea