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

[Andrew Jones <ajones@ventanamicro.com>]

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