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