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

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Date: Thu, 27 Jun 2024 13:06:42 +0200
From: Andrea Parri <parri.andrea@...il.com>
To: 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

> -#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)


> +end:;									\

Why the semicolon?


>  })
>  
>  #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?

  Andrea