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

[Alexandre Ghiti <alex@ghiti.fr>]

On 27/06/2024 15:31, Andrea Parri wrote:
> 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?


Not at all, I overinterpreted your suggestion, I'll restrict this to the 
fully-ordered macro then.

Thanks,


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