113 lines
3.8 KiB
C
113 lines
3.8 KiB
C
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/*
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* Copyright 2013-present Facebook, Inc.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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#pragma once
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#include <atomic>
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#include <cassert>
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#include <chrono>
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#include <cstdint>
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#include <limits>
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#include <type_traits>
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#include <folly/portability/Unistd.h>
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namespace folly {
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namespace detail {
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enum class FutexResult {
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VALUE_CHANGED, /* futex value didn't match expected */
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AWOKEN, /* wakeup by matching futex wake, or spurious wakeup */
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INTERRUPTED, /* wakeup by interrupting signal */
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TIMEDOUT, /* wakeup by expiring deadline */
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};
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/**
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* Futex is an atomic 32 bit unsigned integer that provides access to the
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* futex() syscall on that value. It is templated in such a way that it
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* can interact properly with DeterministicSchedule testing.
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*
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* If you don't know how to use futex(), you probably shouldn't be using
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* this class. Even if you do know how, you should have a good reason
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* (and benchmarks to back you up).
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*
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* Because of the semantics of the futex syscall, the futex family of
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* functions are available as free functions rather than member functions
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*/
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template <template <typename> class Atom = std::atomic>
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using Futex = Atom<std::uint32_t>;
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/**
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* Puts the thread to sleep if this->load() == expected. Returns true when
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* it is returning because it has consumed a wake() event, false for any
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* other return (signal, this->load() != expected, or spurious wakeup).
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*/
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template <typename Futex>
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FutexResult
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futexWait(const Futex* futex, uint32_t expected, uint32_t waitMask = -1);
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/**
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* Similar to futexWait but also accepts a deadline until when the wait call
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* may block.
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*
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* Optimal clock types: std::chrono::system_clock, std::chrono::steady_clock.
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* NOTE: On some systems steady_clock is just an alias for system_clock,
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* and is not actually steady.
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*
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* For any other clock type, now() will be invoked twice.
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*/
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template <
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typename Futex,
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class Clock,
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class Duration = typename Clock::duration>
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FutexResult futexWaitUntil(
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const Futex* futex,
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uint32_t expected,
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std::chrono::time_point<Clock, Duration> const& deadline,
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uint32_t waitMask = -1);
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/**
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* Wakes up to count waiters where (waitMask & wakeMask) != 0, returning the
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* number of awoken threads, or -1 if an error occurred. Note that when
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* constructing a concurrency primitive that can guard its own destruction, it
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* is likely that you will want to ignore EINVAL here (as well as making sure
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* that you never touch the object after performing the memory store that is
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* the linearization point for unlock or control handoff). See
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* https://sourceware.org/bugzilla/show_bug.cgi?id=13690
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*/
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template <typename Futex>
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int futexWake(
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const Futex* futex,
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int count = std::numeric_limits<int>::max(),
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uint32_t wakeMask = -1);
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/** A std::atomic subclass that can be used to force Futex to emulate
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* the underlying futex() syscall. This is primarily useful to test or
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* benchmark the emulated implementation on systems that don't need it. */
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template <typename T>
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struct EmulatedFutexAtomic : public std::atomic<T> {
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EmulatedFutexAtomic() noexcept = default;
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constexpr /* implicit */ EmulatedFutexAtomic(T init) noexcept
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: std::atomic<T>(init) {}
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// It doesn't copy or move
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EmulatedFutexAtomic(EmulatedFutexAtomic&& rhs) = delete;
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};
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} // namespace detail
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} // namespace folly
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#include <folly/detail/Futex-inl.h>
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