Re: [PATCH v8 02/12] arm64: barrier: Support smp_cond_load_relaxed_timeout()

[Will Deacon <will@kernel.org>]

On Sun, Dec 14, 2025 at 08:49:09PM -0800, Ankur Arora wrote:
> Support waiting in smp_cond_load_relaxed_timeout() via
> __cmpwait_relaxed(). To ensure that we wake from waiting in
> WFE periodically and don't block forever if there are no stores
> to ptr, this path is only used when the event-stream is enabled.
> 
> Note that when using __cmpwait_relaxed() we ignore the timeout
> value, allowing an overshoot by upto the event-stream period.
> And, in the unlikely event that the event-stream is unavailable,
> fallback to spin-waiting.
> 
> Also set SMP_TIMEOUT_POLL_COUNT to 1 so we do the time-check in
> each iteration of smp_cond_load_relaxed_timeout().
> 
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Will Deacon <will@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: linux-arm-kernel@lists.infradead.org
> Suggested-by: Will Deacon <will@kernel.org>
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> ---
> 
> Notes:
>    - cpu_poll_relax() now takes an additional parameter.
> 
>    - added a comment detailing why we define SMP_TIMEOUT_POLL_COUNT=1 and
>      how it ties up with smp_cond_load_relaxed_timeout().
> 
>    - explicitly include <asm/vdso/processor.h> for cpu_relax().
> 
>  arch/arm64/include/asm/barrier.h | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
> index 9495c4441a46..6190e178db51 100644
> --- a/arch/arm64/include/asm/barrier.h
> +++ b/arch/arm64/include/asm/barrier.h
> @@ -12,6 +12,7 @@
>  #include <linux/kasan-checks.h>
>  
>  #include <asm/alternative-macros.h>
> +#include <asm/vdso/processor.h>
>  
>  #define __nops(n)	".rept	" #n "\nnop\n.endr\n"
>  #define nops(n)		asm volatile(__nops(n))
> @@ -219,6 +220,26 @@ do {									\
>  	(typeof(*ptr))VAL;						\
>  })
>  
> +/* Re-declared here to avoid include dependency. */
> +extern bool arch_timer_evtstrm_available(void);
> +
> +/*
> + * In the common case, cpu_poll_relax() sits waiting in __cmpwait_relaxed()
> + * for the ptr value to change.
> + *
> + * Since this period is reasonably long, choose SMP_TIMEOUT_POLL_COUNT
> + * to be 1, so smp_cond_load_{relaxed,acquire}_timeout() does a
> + * time-check in each iteration.
> + */
> +#define SMP_TIMEOUT_POLL_COUNT	1
> +
> +#define cpu_poll_relax(ptr, val, timeout_ns) do {			\
> +	if (arch_timer_evtstrm_available())				\
> +		__cmpwait_relaxed(ptr, val);				\
> +	else								\
> +		cpu_relax();						\
> +} while (0)

Acked-by: Will Deacon <will@kernel.org>

Will