linux-kernel - Re: [PATCH v2 01/10] riscv: Implement cmpxchg32/64() using Zacas

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Message-ID: <4008aeca-352f-489e-ba07-7a11f5ab7ccb@ghiti.fr>
Date: Thu, 4 Jul 2024 18:25:41 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Andrea Parri <parri.andrea@...il.com>,
 Alexandre Ghiti <alexghiti@...osinc.com>
Cc: Jonathan Corbet <corbet@....net>, Paul Walmsley
 <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>,
 Albert Ou <aou@...s.berkeley.edu>, Nathan Chancellor <nathan@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>,
 Boqun Feng <boqun.feng@...il.com>, Arnd Bergmann <arnd@...db.de>,
 Leonardo Bras <leobras@...hat.com>, Guo Ren <guoren@...nel.org>,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-riscv@...ts.infradead.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH v2 01/10] riscv: Implement cmpxchg32/64() using Zacas

Hi Andrea,

On 27/06/2024 13:06, Andrea Parri wrote:
>> -#define __arch_cmpxchg(lr_sfx, sc_sfx, prepend, append, r, p, co, o, n)	\
>> +#define __arch_cmpxchg(lr_sfx, sc_cas_sfx, prepend, append, r, p, co, o, n)	\
>>   ({									\
>> +	__label__ zacas, end;						\
>>   	register unsigned int __rc;					\
>>   									\
>> +	if (IS_ENABLED(CONFIG_RISCV_ISA_ZACAS)) {			\
>> +		asm goto(ALTERNATIVE("nop", "j %[zacas]", 0,		\
>> +				     RISCV_ISA_EXT_ZACAS, 1)		\
>> +			 : : : : zacas);				\
>> +	}								\
>> +									\
>>   	__asm__ __volatile__ (						\
>>   		prepend							\
>>   		"0:	lr" lr_sfx " %0, %2\n"				\
>>   		"	bne  %0, %z3, 1f\n"				\
>> -		"	sc" sc_sfx " %1, %z4, %2\n"			\
>> +		"	sc" sc_cas_sfx " %1, %z4, %2\n"			\
>>   		"	bnez %1, 0b\n"					\
>>   		append							\
>>   		"1:\n"							\
>>   		: "=&r" (r), "=&r" (__rc), "+A" (*(p))			\
>>   		: "rJ" (co o), "rJ" (n)					\
>>   		: "memory");						\
>> +	goto end;							\
>> +									\
>> +zacas:									\
>> +	if (IS_ENABLED(CONFIG_RISCV_ISA_ZACAS)) {			\
>> +		__asm__ __volatile__ (					\
>> +			prepend						\
>> +			"	amocas" sc_cas_sfx " %0, %z2, %1\n"	\
>> +			append						\
>> +			: "+&r" (r), "+A" (*(p))			\
>> +			: "rJ" (n)					\
>> +			: "memory");					\
>> +	}								\
> Is this second IS_ENABLED(CONFIG_RISCV_ISA_ZACAS) check actually needed?
> (just wondering - no real objection)

To me yes, otherwise a toolchain without zacas support would fail to 
assemble the amocas instruction.

>
>
>> +end:;									\
> Why the semicolon?

That fixes a clang warning reported by Nathan here: 
https://lore.kernel.org/linux-riscv/20240528193110.GA2196855@thelio-3990X/

>
>
>>   })
>>   
>>   #define _arch_cmpxchg(ptr, old, new, sc_sfx, prepend, append)		\
>> @@ -156,7 +177,7 @@
>>   	__typeof__(ptr) __ptr = (ptr);					\
>>   	__typeof__(*(__ptr)) __old = (old);				\
>>   	__typeof__(*(__ptr)) __new = (new);				\
>> -	__typeof__(*(__ptr)) __ret;					\
>> +	__typeof__(*(__ptr)) __ret = (old);				\
> This is because the compiler doesn't realize __ret is actually
> initialized, right?  IAC, seems a bit unexpected to initialize
> with (old) (which indicates SUCCESS of the CMPXCHG operation);
> how about using (new) for the initialization of __ret instead?
> would (new) still work for you?

But amocas rd register must contain the expected old value in order to 
actually work right?

>
>    Andrea
>
> _______________________________________________
> linux-riscv mailing list
> linux-riscv@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-riscv

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