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

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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

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