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1// -*- C++ -*-2//===----------------------------------------------------------------------===//3//4// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.5// See https://llvm.org/LICENSE.txt for license information.6// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception7//8//===----------------------------------------------------------------------===//9 10#ifndef _LIBCPP___THREAD_POLL_WITH_BACKOFF_H11#define _LIBCPP___THREAD_POLL_WITH_BACKOFF_H12 13#include <__chrono/duration.h>14#include <__chrono/high_resolution_clock.h>15#include <__config>16 17#if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)18# pragma GCC system_header19#endif20 21_LIBCPP_BEGIN_NAMESPACE_STD22 23static _LIBCPP_CONSTEXPR const int __libcpp_polling_count = 64;24 25// Polls a thread for a condition given by a predicate, and backs off based on a backoff policy26// before polling again.27//28// - __poll is the "test function" that should return true if polling succeeded, and false if it failed.29//30// - __backoff is the "backoff policy", which is called with the duration since we started polling. It should31// return false in order to resume polling, and true if polling should stop entirely for some reason.32// In general, backoff policies sleep for some time before returning control to the polling loop.33//34// - __max_elapsed is the maximum duration to try polling for. If the maximum duration is exceeded,35// the polling loop will return false to report a timeout.36template <class _Poll, class _Backoff>37_LIBCPP_HIDE_FROM_ABI bool __libcpp_thread_poll_with_backoff(38 _Poll&& __poll, _Backoff&& __backoff, chrono::nanoseconds __max_elapsed = chrono::nanoseconds::zero()) {39 auto const __start = chrono::high_resolution_clock::now();40 for (int __count = 0;;) {41 if (__poll())42 return true; // __poll completion means success43 if (__count < __libcpp_polling_count) {44 __count += 1;45 continue;46 }47 chrono::nanoseconds const __elapsed = chrono::high_resolution_clock::now() - __start;48 if (__max_elapsed != chrono::nanoseconds::zero() && __max_elapsed < __elapsed)49 return false; // timeout failure50 if (__backoff(__elapsed))51 return false; // __backoff completion means failure52 }53}54 55// A trivial backoff policy that always immediately returns the control to56// the polling loop.57//58// This is not very well-behaved since it will cause the polling loop to spin,59// so this should most likely only be used on single-threaded systems where there60// are no other threads to compete with.61struct __spinning_backoff_policy {62 _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR bool operator()(chrono::nanoseconds const&) const { return false; }63};64 65_LIBCPP_END_NAMESPACE_STD66 67#endif // _LIBCPP___THREAD_POLL_WITH_BACKOFF_H68