Add smp_cond_load_relaxed_timewait(), a timed variant of
smp_cond_load_relaxed().

This uses __cmpwait_relaxed() to do the actual waiting, with the
event-stream guaranteeing that we wake up from WFE periodically
and not block forever in case there are no stores to the cacheline.

For cases when the event-stream is unavailable, fallback to
spin-waiting.

Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
---
 arch/arm64/include/asm/barrier.h | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index f5801b0ba9e9..9b29abc212db 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -219,6 +219,28 @@ do {									\
 	(typeof(*ptr))VAL;						\
 })
 
+extern bool arch_timer_evtstrm_available(void);
+
+#define smp_cond_load_relaxed_timewait(ptr, cond_expr, time_check_expr)	\
+({									\
+	typeof(ptr) __PTR = (ptr);					\
+	__unqual_scalar_typeof(*ptr) VAL;				\
+	bool __wfe = arch_timer_evtstrm_available();			\
+									\
+	for (;;) {							\
+		VAL = READ_ONCE(*__PTR);				\
+		if (cond_expr)						\
+			break;						\
+		if (time_check_expr)					\
+			break;						\
+		if (likely(__wfe))					\
+			__cmpwait_relaxed(__PTR, VAL);			\
+		else							\
+			cpu_relax();					\
+	}								\
+	(typeof(*ptr)) VAL;						\
+})
+
 #include <asm-generic/barrier.h>
 
 #endif	/* __ASSEMBLY__ */
-- 
2.31.1

On Fri, Aug 29, 2025 at 01:07:32AM -0700, Ankur Arora wrote:
> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> index f5801b0ba9e9..9b29abc212db 100644
> --- a/arch/arm64/include/asm/barrier.h
> +++ b/arch/arm64/include/asm/barrier.h
> @@ -219,6 +219,28 @@ do {									\
>  	(typeof(*ptr))VAL;						\
>  })
>  
> +extern bool arch_timer_evtstrm_available(void);

In theory, this doesn't work if CONFIG_ARM_ARCH_TIMER is disabled,
though that's not the case for arm64. Maybe add a short comment that's
re-declared here to avoid include dependencies.

> +
> +#define smp_cond_load_relaxed_timewait(ptr, cond_expr, time_check_expr)	\
> +({									\
> +	typeof(ptr) __PTR = (ptr);					\
> +	__unqual_scalar_typeof(*ptr) VAL;				\
> +	bool __wfe = arch_timer_evtstrm_available();			\
> +									\
> +	for (;;) {							\
> +		VAL = READ_ONCE(*__PTR);				\
> +		if (cond_expr)						\
> +			break;						\
> +		if (time_check_expr)					\
> +			break;						\
> +		if (likely(__wfe))					\
> +			__cmpwait_relaxed(__PTR, VAL);			\
> +		else							\
> +			cpu_relax();					\
> +	}								\
> +	(typeof(*ptr)) VAL;						\
> +})
> +
>  #include <asm-generic/barrier.h>
>  
>  #endif	/* __ASSEMBLY__ */

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

Catalin Marinas <catalin.marinas@arm.com> writes:

> On Fri, Aug 29, 2025 at 01:07:32AM -0700, Ankur Arora wrote:
>> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
>> index f5801b0ba9e9..9b29abc212db 100644
>> --- a/arch/arm64/include/asm/barrier.h
>> +++ b/arch/arm64/include/asm/barrier.h
>> @@ -219,6 +219,28 @@ do {									\
>>  	(typeof(*ptr))VAL;						\
>>  })
>>
>> +extern bool arch_timer_evtstrm_available(void);
>
> In theory, this doesn't work if CONFIG_ARM_ARCH_TIMER is disabled,
> though that's not the case for arm64. Maybe add a short comment that's
> re-declared here to avoid include dependencies.

Will add.

Thanks
Ankur

>> +
>> +#define smp_cond_load_relaxed_timewait(ptr, cond_expr, time_check_expr)	\
>> +({									\
>> +	typeof(ptr) __PTR = (ptr);					\
>> +	__unqual_scalar_typeof(*ptr) VAL;				\
>> +	bool __wfe = arch_timer_evtstrm_available();			\
>> +									\
>> +	for (;;) {							\
>> +		VAL = READ_ONCE(*__PTR);				\
>> +		if (cond_expr)						\
>> +			break;						\
>> +		if (time_check_expr)					\
>> +			break;						\
>> +		if (likely(__wfe))					\
>> +			__cmpwait_relaxed(__PTR, VAL);			\
>> +		else							\
>> +			cpu_relax();					\
>> +	}								\
>> +	(typeof(*ptr)) VAL;						\
>> +})
>> +
>>  #include <asm-generic/barrier.h>
>>
>>  #endif	/* __ASSEMBLY__ */
>
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>


--
ankur