Re: [PATCH v2 04/10] riscv: Improve amocas.X use in cmpxchg()

Andrea Parri
2024-06-27T13:31:43Z

On Wed, Jun 26, 2024 at 03:03:41PM +0200, Alexandre Ghiti wrote:
> cmpxchg() uses amocas.X instructions from Zacas and Zabha but still uses
> the LR/SC acquire/release semantics which require barriers.
>
> Let's improve that by using proper amocas acquire/release semantics in
> order to avoid any of those barriers.

I can't really parse this changelog...

> Suggested-by: Andrea Parri <andrea@rivosinc.com>
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> ---
> arch/riscv/include/asm/cmpxchg.h | 60 ++++++++++++++++++--------------
> 1 file changed, 33 insertions(+), 27 deletions(-)
>
> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> index b9a3fdcec919..3c65b00a0d36 100644
> --- a/arch/riscv/include/asm/cmpxchg.h
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -105,7 +105,9 @@
> * indicated by comparing RETURN with OLD.
> */
>
> -#define __arch_cmpxchg_masked(sc_sfx, cas_sfx, prepend, append, r, p, o, n) \
> +#define __arch_cmpxchg_masked(sc_sfx, cas_sfx, \
> + sc_prepend, sc_append, \
> + r, p, o, n) \
> ({ \
> __label__ no_zacas, zabha, end; \
> \
> @@ -129,7 +131,7 @@ no_zacas:; \
> ulong __rc; \
> \
> __asm__ __volatile__ ( \
> - prepend \
> + sc_prepend \
> "0: lr.w %0, %2\n" \
> " and %1, %0, %z5\n" \
> " bne %1, %z3, 1f\n" \
> @@ -137,7 +139,7 @@ no_zacas:; \
> " or %1, %1, %z4\n" \
> " sc.w" sc_sfx " %1, %1, %2\n" \
> " bnez %1, 0b\n" \
> - append \
> + sc_append \
> "1:\n" \
> : "=&r" (__retx), "=&r" (__rc), "+A" (*(__ptr32b)) \
> : "rJ" ((long)__oldx), "rJ" (__newx), \
> @@ -150,9 +152,7 @@ no_zacas:; \
> zabha: \
> if (IS_ENABLED(CONFIG_RISCV_ISA_ZABHA)) { \
> __asm__ __volatile__ ( \
> - prepend \
> " amocas" cas_sfx " %0, %z2, %1\n" \
> - append \
> : "+&r" (r), "+A" (*(p)) \
> : "rJ" (n) \
> : "memory"); \
> @@ -160,7 +160,9 @@ zabha: \
> end:; \
> })
>
> -#define __arch_cmpxchg(lr_sfx, sc_cas_sfx, prepend, append, r, p, co, o, n) \
> +#define __arch_cmpxchg(lr_sfx, sc_sfx, cas_sfx, \
> + sc_prepend, sc_append, \
> + r, p, co, o, n) \
> ({ \
> __label__ zacas, end; \
> register unsigned int __rc; \
> @@ -172,12 +174,12 @@ end:; \
> } \
> \
> __asm__ __volatile__ ( \
> - prepend \
> + sc_prepend \
> "0: lr" lr_sfx " %0, %2\n" \
> " bne %0, %z3, 1f\n" \
> - " sc" sc_cas_sfx " %1, %z4, %2\n" \
> + " sc" sc_sfx " %1, %z4, %2\n" \
> " bnez %1, 0b\n" \
> - append \
> + sc_append \
> "1:\n" \
> : "=&r" (r), "=&r" (__rc), "+A" (*(p)) \
> : "rJ" (co o), "rJ" (n) \
> @@ -187,9 +189,7 @@ end:; \
> zacas: \
> if (IS_ENABLED(CONFIG_RISCV_ISA_ZACAS)) { \
> __asm__ __volatile__ ( \
> - prepend \
> - " amocas" sc_cas_sfx " %0, %z2, %1\n" \
> - append \
> + " amocas" cas_sfx " %0, %z2, %1\n" \
> : "+&r" (r), "+A" (*(p)) \
> : "rJ" (n) \
> : "memory"); \
> @@ -197,7 +197,8 @@ zacas: \
> end:; \
> })
>
> -#define _arch_cmpxchg(ptr, old, new, sc_sfx, prepend, append) \
> +#define _arch_cmpxchg(ptr, old, new, sc_sfx, cas_sfx, \
> + sc_prepend, sc_append) \
> ({ \
> __typeof__(ptr) __ptr = (ptr); \
> __typeof__(*(__ptr)) __old = (old); \
> @@ -206,22 +207,24 @@ end:; \
> \
> switch (sizeof(*__ptr)) { \
> case 1: \
> - __arch_cmpxchg_masked(sc_sfx, ".b" sc_sfx, \
> - prepend, append, \
> - __ret, __ptr, __old, __new); \
> + __arch_cmpxchg_masked(sc_sfx, ".b" cas_sfx, \
> + sc_prepend, sc_append, \
> + __ret, __ptr, __old, __new); \
> break; \
> case 2: \
> - __arch_cmpxchg_masked(sc_sfx, ".h" sc_sfx, \
> - prepend, append, \
> - __ret, __ptr, __old, __new); \
> + __arch_cmpxchg_masked(sc_sfx, ".h" cas_sfx, \
> + sc_prepend, sc_append, \
> + __ret, __ptr, __old, __new); \
> break; \
> case 4: \
> - __arch_cmpxchg(".w", ".w" sc_sfx, prepend, append, \
> - __ret, __ptr, (long), __old, __new); \
> + __arch_cmpxchg(".w", ".w" sc_sfx, ".w" cas_sfx, \
> + sc_prepend, sc_append, \
> + __ret, __ptr, (long), __old, __new); \
> break; \
> case 8: \
> - __arch_cmpxchg(".d", ".d" sc_sfx, prepend, append, \
> - __ret, __ptr, /**/, __old, __new); \
> + __arch_cmpxchg(".d", ".d" sc_sfx, ".d" cas_sfx, \
> + sc_prepend, sc_append, \
> + __ret, __ptr, /**/, __old, __new); \
> break; \
> default: \
> BUILD_BUG(); \
> @@ -230,16 +233,19 @@ end:; \
> })
>
> #define arch_cmpxchg_relaxed(ptr, o, n) \
> - _arch_cmpxchg((ptr), (o), (n), "", "", "")
> + _arch_cmpxchg((ptr), (o), (n), "", "", "", "")
>
> #define arch_cmpxchg_acquire(ptr, o, n) \
> - _arch_cmpxchg((ptr), (o), (n), "", "", RISCV_ACQUIRE_BARRIER)
> + _arch_cmpxchg((ptr), (o), (n), "", ".aq", \
> + "", RISCV_ACQUIRE_BARRIER)
>
> #define arch_cmpxchg_release(ptr, o, n) \
> - _arch_cmpxchg((ptr), (o), (n), "", RISCV_RELEASE_BARRIER, "")
> + _arch_cmpxchg((ptr), (o), (n), "", ".rl", \
> + RISCV_RELEASE_BARRIER, "")
>
> #define arch_cmpxchg(ptr, o, n) \
> - _arch_cmpxchg((ptr), (o), (n), ".rl", "", " fence rw, rw\n")
> + _arch_cmpxchg((ptr), (o), (n), ".rl", ".aqrl", \
> + "", RISCV_FULL_BARRIER)

... but this is not what I suggested: my suggestion [1] was about (limited
to) the fully-ordered macro arch_cmpxchg(). In fact, I've recently raised
some concern about similar changes to the acquire/release macros, cf. [2].

Any particular reasons for doing this?

Andrea

[1] https://lore.kernel.org/lkml/ZlYff9x12FICHoP0@andrea/
[2] https://lore.kernel.org/lkml/20240505123340.38495-1-puranjay@kernel.org/