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evvFSjUi11CQXjD3aviz6JNOpGyRw--1 David Laight writes: > On Sun, 8 Feb 2026 18:31:42 -0800 > Ankur Arora wrote: > >> Add smp_cond_load_relaxed_timeout(), which extends >> smp_cond_load_relaxed() to allow waiting for a duration. >> >> We loop around waiting for the condition variable to change while >> peridically doing a time-check. The loop uses cpu_poll_relax() to slow >> down the busy-waiting, which, unless overridden by the architecture >> code, amounts to a cpu_relax(). >> >> Note that there are two ways for the time-check to fail: the usual >> timeout case or, @time_expr_ns returning an invalid value (negative >> or zero). The second failure mode allows for clocks attached to the >> clock-domain of @cond_expr, which might cease to operate meaningfully >> once some state internal to @cond_expr has changed. >> >> Evaluation of @time_expr_ns: in the fastpath we want to keep the >> performance close to smp_cond_load_relaxed(). To do that we defer >> evaluation of the potentially costly @time_expr_ns to when we hit >> the slowpath. >> >> This also means that there will always be some hardware dependent >> duration that has passed in cpu_poll_relax() iterations at the time of >> first evaluation. Additionally cpu_poll_relax() is not guaranteed to >> return at timeout boundary. In sum, expect timeout overshoot when we >> exit due to expiration of the timeout. >> >> The number of spin iterations before time-check, SMP_TIMEOUT_POLL_COUNT >> is chosen to be 200 by default. With a cpu_poll_relax() iteration >> taking ~20-30 cycles (measured on a variety of x86 platforms), we expect >> a tim-check every ~4000-6000 cycles. > ^ time-check Ugh. Thanks. > Plus the cost of evaluating cond_expr 200 times. > I guess that isn't expected to contain a PCIe read :-) :). Good point. I'll see if I can add something like "when polling on a memory address". Ankur >> >> The outer limit of the overshoot is double that when working with the >> parameters above. This might be higher or lower depending on the >> implementation of cpu_poll_relax() across architectures. >> >> Lastly, config option ARCH_HAS_CPU_RELAX indicates availability of a >> cpu_poll_relax() that is cheaper than polling. This might be relevant >> for cases with a prolonged timeout. >> >> Cc: Arnd Bergmann >> Cc: Will Deacon >> Cc: Catalin Marinas >> Cc: Peter Zijlstra >> Cc: linux-arch@vger.kernel.org >> Signed-off-by: Ankur Arora >> --- >> Notes: >> - Defer evaluation of @time_expr_ns to when we hit the slowpath. >> - This also helps get rid of the labelled gotos which were used to >> handle the early failure case (since now there's no early init >> to be concerned with.) >> - Add a comment mentioning that the cpu_poll_relax() implementation >> is better than polling if ARCH_HAS_CPU_RELAX. >> >> include/asm-generic/barrier.h | 72 +++++++++++++++++++++++++++++++++++ >> 1 file changed, 72 insertions(+) >> >> diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h >> index d4f581c1e21d..2738fe35c1df 100644 >> --- a/include/asm-generic/barrier.h >> +++ b/include/asm-generic/barrier.h >> @@ -273,6 +273,68 @@ do { \ >> }) >> #endif >> >> +/* >> + * Number of times we iterate in the loop before doing the time check. >> + */ >> +#ifndef SMP_TIMEOUT_POLL_COUNT >> +#define SMP_TIMEOUT_POLL_COUNT 200 >> +#endif >> + >> +/* >> + * Platforms with ARCH_HAS_CPU_RELAX have a cpu_poll_relax() implementation >> + * that is expected to be cheaper (lower power) than pure polling. >> + */ >> +#ifndef cpu_poll_relax >> +#define cpu_poll_relax(ptr, val, timeout_ns) cpu_relax() >> +#endif >> + >> +/** >> + * 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_ns: expression that evaluates to monotonic time (in ns) or, >> + * on failure, returns a negative value. >> + * @timeout_ns: timeout value in ns >> + * Both of the above are assumed to be compatible with s64; the signed >> + * value is used to handle the failure case in @time_expr_ns. >> + * >> + * Equivalent to using READ_ONCE() on the condition variable. >> + * >> + * Callers that expect to wait for prolonged durations might want to >> + * take into account the availability of ARCH_HAS_CPU_RELAX. >> + */ >> +#ifndef smp_cond_load_relaxed_timeout >> +#define smp_cond_load_relaxed_timeout(ptr, cond_expr, \ >> + time_expr_ns, timeout_ns) \ >> +({ \ >> + typeof(ptr) __PTR = (ptr); \ >> + __unqual_scalar_typeof(*ptr) VAL; \ >> + u32 __n = 0, __spin = SMP_TIMEOUT_POLL_COUNT; \ >> + s64 __timeout = (s64)timeout_ns; \ >> + s64 __time_now, __time_end = 0; \ >> + \ >> + for (;;) { \ >> + VAL = READ_ONCE(*__PTR); \ >> + if (cond_expr) \ >> + break; \ >> + cpu_poll_relax(__PTR, VAL, (u64)__timeout); \ >> + if (++__n < __spin) \ >> + continue; \ >> + __time_now = (s64)(time_expr_ns); \ >> + if (unlikely(__time_end == 0)) \ >> + __time_end = __time_now + __timeout; \ >> + __timeout = __time_end - __time_now; \ >> + if (__time_now <= 0 || __timeout <= 0) { \ >> + VAL = READ_ONCE(*__PTR); \ >> + break; \ >> + } \ >> + __n = 0; \ >> + } \ >> + (typeof(*ptr))VAL; \ >> +}) >> +#endif >> + >> /* >> * pmem_wmb() ensures that all stores for which the modification >> * are written to persistent storage by preceding instructions have -- ankur