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

Alexandre Ghiti
2024-07-04T16:40:29Z

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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