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

Catalin Marinas
2025-11-04T13:55:42Z

On Mon, Nov 03, 2025 at 01:00:33PM -0800, Ankur Arora wrote:
> /**
> * smp_cond_load_relaxed_timeout() - (Spin) wait for cond with no ordering
> * guarantees until a timeout expires.
> * @ptr: pointer to the variable to wait on
> * @cond: boolean expression to wait for
> * @time_expr: time expression in caller's preferred clock
> * @time_end: end time in nanosecond (compared against time_expr;
> * might also be used for setting up a future event.)
> *
> * Equivalent to using READ_ONCE() on the condition variable.
> *
> * Note that the expiration of the timeout might have an architecture specific
> * delay.
> */
> #ifndef smp_cond_load_relaxed_timeout
> #define smp_cond_load_relaxed_timeout(ptr, cond_expr, time_expr, time_end_ns) \
> ({ \
> typeof(ptr) __PTR = (ptr); \
> __unqual_scalar_typeof(*ptr) VAL; \
> u32 __n = 0, __spin = SMP_TIMEOUT_POLL_COUNT; \
> u64 __time_end_ns = (time_end_ns); \
> \
> for (;;) { \
> VAL = READ_ONCE(*__PTR); \
> if (cond_expr) \
> break; \
> cpu_poll_relax(__PTR, VAL, __time_end_ns); \

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 ;)).

> if (++__n < __spin) \
> continue; \
> if ((time_expr) >= __time_end_ns) { \
> VAL = READ_ONCE(*__PTR); \
> break; \
> } \
> __n = 0; \
> } \
> (typeof(*ptr))VAL; \
> })
> #endif

--
Catalin