Re: [RESEND PATCH v7 2/7] arm64: barrier: Support smp_cond_load_relaxed_timeout()

Arnd Bergmann
2025-11-05T10:41:13Z

On Wed, Nov 5, 2025, at 09:27, Ankur Arora wrote:
> Catalin Marinas <catalin.marinas@arm.com> writes:
>> On Mon, Nov 03, 2025 at 01:00:33PM -0800, Ankur Arora wrote:
>> With time_end_ns being passed to cpu_poll_relax(), we assume that this
>> is always the absolute time. Do we still need time_expr in this case?
>> It works for WFET as long as we can map this time_end_ns onto the
>> hardware CNTVCT.
>>
>> Alternatively, we could pass something like remaining_ns, though not
>> sure how smp_cond_load_relaxed_timeout() can decide to spin before
>> checking time_expr again (we probably went over this in the past two
>> years ;)).
>
> I'm sure it is in there somewhere :).
> This one?: https://lore.kernel.org/lkml/aJy414YufthzC1nv@arm.com/.
> Though the whole wait_policy thing confused the issue somewhat there.
>
> Though that problem exists for both remaining_ns and for time_end_ns
> with WFE. I think we are fine so long as SMP_TIMEOUT_POLL_COUNT is
> defined to be 1.

We need to be careful with the definition of the time_expr() if
cpu_poll_relax requires the absolute time in CNTVCT domain.
We're probably fine here, but it feels like a bit of a layering
violation. I think passing relative time into it would be cleaner
because it avoids the ambiguity it but probably requires an extra
access to the timer register that is hopefully fast on arm64.

I'm ok with either way.

> For now, I think it makes sense to always pass the absolute deadline
> even if the caller uses remaining_ns. So:
>
> #define smp_cond_load_relaxed_timeout(ptr, cond_expr, time_expr, remaining_ns) \
> ({ \
> typeof(ptr) __PTR = (ptr); \
> __unqual_scalar_typeof(*ptr) VAL; \
> u32 __n = 0, __spin = SMP_TIMEOUT_POLL_COUNT; \
> u64 __time_start_ns = (time_expr); \
> s64 __time_end_ns = __time_start_ns + (remaining_ns); \
> \
> for (;;) { \
> VAL = READ_ONCE(*__PTR); \
> if (cond_expr) \
> break; \
> cpu_poll_relax(__PTR, VAL, __time_end_ns); \

This looks perfectly fine to me, thanks for the update!

Arnd