Re: [PATCH v4 06/13] riscv: Improve zacas fully-ordered cmpxchg()

Andrew Jones
2024-07-31T09:59:05Z

On Wed, Jul 31, 2024 at 09:23:58AM GMT, Alexandre Ghiti wrote:
> The current fully-ordered cmpxchgXX() implementation results in:
>
> amocas.X.rl a5,a4,(s1)
> fence rw,rw
>
> This provides enough sync but we can actually use the following better
> mapping instead:
>
> amocas.X.aqrl a5,a4,(s1)

We won't get release semantics if the exchange fails. Does that matter?

>
> Suggested-by: Andrea Parri <andrea@rivosinc.com>
> Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> ---
> arch/riscv/include/asm/cmpxchg.h | 72 +++++++++++++++++++-------------
> 1 file changed, 44 insertions(+), 28 deletions(-)
>
> diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> index ebcd4a30ae60..391730367213 100644
> --- a/arch/riscv/include/asm/cmpxchg.h
> +++ b/arch/riscv/include/asm/cmpxchg.h
> @@ -107,8 +107,10 @@
> * store NEW in MEM. Return the initial value in MEM. Success is
> * 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, \
> + cas_prepend, cas_append, \
> + r, p, o, n) \
> ({ \
> if (IS_ENABLED(CONFIG_RISCV_ISA_ZABHA) && \
> IS_ENABLED(CONFIG_RISCV_ISA_ZACAS) && \
> @@ -117,9 +119,9 @@
> r = o; \
> \
> __asm__ __volatile__ ( \
> - prepend \
> + cas_prepend \
> " amocas" cas_sfx " %0, %z2, %1\n" \
> - append \
> + cas_append \
> : "+&r" (r), "+A" (*(p)) \
> : "rJ" (n) \
> : "memory"); \
> @@ -134,7 +136,7 @@
> ulong __rc; \
> \
> __asm__ __volatile__ ( \
> - prepend \
> + sc_prepend \
> "0: lr.w %0, %2\n" \
> " and %1, %0, %z5\n" \
> " bne %1, %z3, 1f\n" \
> @@ -142,7 +144,7 @@
> " 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), \
> @@ -153,16 +155,19 @@
> } \
> })
>
> -#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, \
> + cas_prepend, cas_append, \
> + r, p, co, o, n) \
> ({ \
> if (IS_ENABLED(CONFIG_RISCV_ISA_ZACAS) && \
> riscv_has_extension_unlikely(RISCV_ISA_EXT_ZACAS)) { \
> r = o; \
> \
> __asm__ __volatile__ ( \
> - prepend \
> - " amocas" sc_cas_sfx " %0, %z2, %1\n" \
> - append \
> + cas_prepend \
> + " amocas" cas_sfx " %0, %z2, %1\n" \
> + cas_append \
> : "+&r" (r), "+A" (*(p)) \
> : "rJ" (n) \
> : "memory"); \
> @@ -170,12 +175,12 @@
> register unsigned int __rc; \
> \
> __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" \

nit: If patch3 hadn't renamed sc_sfx to sc_cas_sfx then we wouldn't
need to rename it again now.

> " bnez %1, 0b\n" \
> - append \
> + sc_append \
> "1:\n" \
> : "=&r" (r), "=&r" (__rc), "+A" (*(p)) \
> : "rJ" (co o), "rJ" (n) \
> @@ -183,7 +188,9 @@
> } \
> })
>
> -#define _arch_cmpxchg(ptr, old, new, sc_cas_sfx, prepend, append) \
> +#define _arch_cmpxchg(ptr, old, new, sc_sfx, cas_sfx, \
> + sc_prepend, sc_append, \
> + cas_prepend, cas_append) \
> ({ \
> __typeof__(ptr) __ptr = (ptr); \
> __typeof__(*(__ptr)) __old = (old); \
> @@ -192,22 +199,28 @@
> \
> switch (sizeof(*__ptr)) { \
> case 1: \
> - __arch_cmpxchg_masked(sc_cas_sfx, ".b" sc_cas_sfx, \
> - prepend, append, \
> - __ret, __ptr, __old, __new); \
> + __arch_cmpxchg_masked(sc_sfx, ".b" cas_sfx, \
> + sc_prepend, sc_append, \
> + cas_prepend, cas_append, \
> + __ret, __ptr, __old, __new); \
> break; \
> case 2: \
> - __arch_cmpxchg_masked(sc_cas_sfx, ".h" sc_cas_sfx, \
> - prepend, append, \
> - __ret, __ptr, __old, __new); \
> + __arch_cmpxchg_masked(sc_sfx, ".h" cas_sfx, \
> + sc_prepend, sc_append, \
> + cas_prepend, cas_append, \
> + __ret, __ptr, __old, __new); \
> break; \
> case 4: \
> - __arch_cmpxchg(".w", ".w" sc_cas_sfx, prepend, append, \
> - __ret, __ptr, (long), __old, __new); \
> + __arch_cmpxchg(".w", ".w" sc_sfx, ".w" cas_sfx, \
> + sc_prepend, sc_append, \
> + cas_prepend, cas_append, \
> + __ret, __ptr, (long), __old, __new); \
> break; \
> case 8: \
> - __arch_cmpxchg(".d", ".d" sc_cas_sfx, prepend, append, \
> - __ret, __ptr, /**/, __old, __new); \
> + __arch_cmpxchg(".d", ".d" sc_sfx, ".d" cas_sfx, \
> + sc_prepend, sc_append, \
> + cas_prepend, cas_append, \
> + __ret, __ptr, /**/, __old, __new); \
> break; \
> default: \
> BUILD_BUG(); \
> @@ -216,16 +229,19 @@
> })
>
> #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), "", "", \
> + "", RISCV_ACQUIRE_BARRIER, "", RISCV_ACQUIRE_BARRIER)
>
> #define arch_cmpxchg_release(ptr, o, n) \
> - _arch_cmpxchg((ptr), (o), (n), "", RISCV_RELEASE_BARRIER, "")
> + _arch_cmpxchg((ptr), (o), (n), "", "", \
> + RISCV_RELEASE_BARRIER, "", 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, "", "")

These aren't the easiest things to read, but I can't think of a way to
improve it other than maybe some macro annotations. E.g.

#define SC_SFX(x) x
#define CAS_SFX(x) x
#define SC_PREPEND(x) x
#define SC_APPEND(x) x
#define CAS_PREPEND(x) x
#define CAS_APPEND(x) x

#define arch_cmpxchg(ptr, o, n) \
_arch_cmpxchg(ptr, o, n, \
SC_SFX(".rl"), CAS_SFX(".aqrl"), \
SC_PREPEND(""), SC_APPEND(RISCV_FULL_BARRIER), \
CAS_PREPEND(""), CAS_APPEND(""))

>
> #define arch_cmpxchg_local(ptr, o, n) \
> arch_cmpxchg_relaxed((ptr), (o), (n))
> --
> 2.39.2
>

Thanks,
drew