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

Alexandre Ghiti
2024-07-04T16:25:46Z

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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