Re: [PATCH 4/4] arm64: barrier: Add smp_cond_load_acquire_timewait()

Ankur Arora
2025-02-18T21:45:22Z

Okanovic, Haris <harisokn@amazon.com> writes:

> On Mon, 2025-02-03 at 13:49 -0800, Ankur Arora wrote:
>> CAUTION: This email originated from outside of the organization. Do not click links or open attachments unless you can confirm the sender and know the content is safe.
>>
>>
>>
>> Add smp_cond_load_acquire_timewait(). This is substantially similar
>> to smp_cond_load_acquire() where we use a load-acquire in the loop
>> and avoid an smp_rmb() later.
>>
>> To handle the unlikely case of the event-stream being unavailable,
>> keep the implementation simple by falling back to the generic
>> __smp_cond_load_relaxed_spinwait() with an smp_rmb() to follow
>> (via smp_acquire__after_ctrl_dep().)
>>
>> Cc: Will Deacon <will@kernel.org>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: linux-arm-kernel@lists.infradead.org
>> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
>> ---
>> arch/arm64/include/asm/barrier.h | 36 ++++++++++++++++++++++++++++++++
>> 1 file changed, 36 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
>> index 25721275a5a2..22d9291aee8d 100644
>> --- a/arch/arm64/include/asm/barrier.h
>> +++ b/arch/arm64/include/asm/barrier.h
>> @@ -232,6 +232,22 @@ do { \
>> (typeof(*ptr))VAL; \
>> })
>>
>> +#define __smp_cond_load_acquire_timewait(ptr, cond_expr, \
>> + time_expr_ns, time_limit_ns) \
>> +({ \
>> + typeof(ptr) __PTR = (ptr); \
>> + __unqual_scalar_typeof(*ptr) VAL; \
>> + for (;;) { \
>> + VAL = smp_load_acquire(__PTR); \
>> + if (cond_expr) \
>> + break; \
>> + __cmpwait_relaxed(__PTR, VAL); \
>> + if ((time_expr_ns) >= (time_limit_ns)) \
>> + break; \
>> + } \
>> + (typeof(*ptr))VAL; \
>> +})
>> +
>> /*
>> * For the unlikely case that the event-stream is unavailable,
>> * ward off the possibility of waiting forever by falling back
>> @@ -254,6 +270,26 @@ do { \
>> (typeof(*ptr))_val; \
>> })
>>
>> +#define smp_cond_load_acquire_timewait(ptr, cond_expr, \
>> + time_expr_ns, time_limit_ns) \
>> +({ \
>> + __unqual_scalar_typeof(*ptr) _val; \
>> + int __wfe = arch_timer_evtstrm_available(); \
>> + \
>> + if (likely(__wfe)) { \
>> + _val = __smp_cond_load_acquire_timewait(ptr, cond_expr, \
>> + time_expr_ns, \
>> + time_limit_ns); \
>> + } else { \
>> + _val = __smp_cond_load_relaxed_spinwait(ptr, cond_expr, \
>> + time_expr_ns, \
>> + time_limit_ns); \
>> + smp_acquire__after_ctrl_dep(); \
>> + } \
>> + (typeof(*ptr))_val; \
>> +})
>> +
>> +
>> #include <asm-generic/barrier.h>
>>
>> #endif /* __ASSEMBLY__ */
>> --
>> 2.43.5
>
> Tested both relaxed and acquire variants on AWS Graviton (ARM64
> Neoverse V1) with your V9 haltpoll changes, atop master 128c8f96eb.
>
> Reviewed-by: Haris Okanovic <harisokn@amazon.com>
> Tested-by: Haris Okanovic <harisokn@amazon.com>

That's great. Thanks Haris.

--
ankur