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53FMSXOjMjNbS7DuwclncFqB2LmX9kTp X-Proofpoint-GUID: 53FMSXOjMjNbS7DuwclncFqB2LmX9kTp 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-wait, 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 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 -- to fail. Evaluation of @time_expr_ns: in the fastpath we want to keep the performance close to smp_cond_load_relaxed(). So defer evaluation of the potentially costly @time_expr_ns to 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 time-check every ~4000-6000 cycles. 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 long timeout. Cc: Arnd Bergmann Cc: Will Deacon Cc: Catalin Marinas Cc: Peter Zijlstra Cc: linux-arch@vger.kernel.org Reviewed-by: Catalin Marinas Signed-off-by: Ankur Arora --- Note: addresses a few Sashiko comments from [1]. We leave unaddressed a couple of potential timeout range issues (around S64_MAX, or during early boot). I had proposed a version earlier that would address those in [2]. Since then, however, I've come to the view that these issues are best handled in code review instead of overcomplicating the implementation. [1] https://sashiko.dev/#/patchset/20260408122538.3610871-1-ankur.a.arora%40oracle.com [2] https://lore.kernel.org/lkml/874iklm1uy.fsf@oracle.com/ --- include/asm-generic/barrier.h | 69 +++++++++++++++++++++++++++++++++++ 1 file changed, 69 insertions(+) diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h index d4f581c1e21d..c56df9513a08 100644 --- a/include/asm-generic/barrier.h +++ b/include/asm-generic/barrier.h @@ -273,6 +273,75 @@ do { \ }) #endif +/* + * Number of times we iterate in the loop before doing the time check. + * Note that the iteration count assumes that the loop condition is + * relatively cheap. + */ +#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_expr: 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. + * + * Note that @ptr is expected to point to a memory address. Using this + * interface with MMIO will be slower (since SMP_TIMEOUT_POLL_COUNT is + * tuned for memory) and might also break in interesting architecture + * dependent ways. + */ +#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 __count = 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 (++__count < __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; \ + } \ + __count = 0; \ + } \ + (typeof(*(ptr)))VAL; \ +}) +#endif + /* * pmem_wmb() ensures that all stores for which the modification * are written to persistent storage by preceding instructions have -- 2.31.1