2 * Copyright 2014 Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
24 #include <boost/noncopyable.hpp>
26 namespace folly { namespace detail {
28 enum class FutexResult {
29 VALUE_CHANGED, /* Futex value didn't match expected */
30 AWOKEN, /* futex wait matched with a futex wake */
31 INTERRUPTED, /* Spurious wake-up or signal caused futex wait failure */
36 * Futex is an atomic 32 bit unsigned integer that provides access to the
37 * futex() syscall on that value. It is templated in such a way that it
38 * can interact properly with DeterministicSchedule testing.
40 * If you don't know how to use futex(), you probably shouldn't be using
41 * this class. Even if you do know how, you should have a good reason
42 * (and benchmarks to back you up).
44 template <template <typename> class Atom = std::atomic>
45 struct Futex : Atom<uint32_t>, boost::noncopyable {
47 explicit Futex(uint32_t init = 0) : Atom<uint32_t>(init) {}
49 /** Puts the thread to sleep if this->load() == expected. Returns true when
50 * it is returning because it has consumed a wake() event, false for any
51 * other return (signal, this->load() != expected, or spurious wakeup). */
52 bool futexWait(uint32_t expected, uint32_t waitMask = -1) {
53 auto rv = futexWaitImpl(expected, nullptr, nullptr, waitMask);
54 assert(rv != FutexResult::TIMEDOUT);
55 return rv == FutexResult::AWOKEN;
58 /** Similar to futexWait but also accepts a timeout that gives the time until
59 * when the call can block (time is the absolute time i.e time since epoch).
60 * Allowed clock types: std::chrono::system_clock, std::chrono::steady_clock.
61 * Returns one of FutexResult values.
63 * NOTE: On some systems steady_clock is just an alias for system_clock,
64 * and is not actually steady.*/
65 template <class Clock, class Duration = typename Clock::duration>
66 FutexResult futexWaitUntil(
68 const std::chrono::time_point<Clock, Duration>& absTime,
69 uint32_t waitMask = -1) {
70 using std::chrono::duration_cast;
71 using std::chrono::nanoseconds;
72 using std::chrono::seconds;
73 using std::chrono::steady_clock;
74 using std::chrono::system_clock;
75 using std::chrono::time_point;
78 (std::is_same<Clock, system_clock>::value ||
79 std::is_same<Clock, steady_clock>::value),
80 "futexWaitUntil only knows std::chrono::{system_clock,steady_clock}");
81 assert((std::is_same<Clock, system_clock>::value) || Clock::is_steady);
83 auto duration = absTime.time_since_epoch();
84 if (std::is_same<Clock, system_clock>::value) {
85 time_point<system_clock> absSystemTime(duration);
86 return futexWaitImpl(expected, &absSystemTime, nullptr, waitMask);
88 time_point<steady_clock> absSteadyTime(duration);
89 return futexWaitImpl(expected, nullptr, &absSteadyTime, waitMask);
93 /** Wakens up to count waiters where (waitMask & wakeMask) != 0,
94 * returning the number of awoken threads. */
95 int futexWake(int count = std::numeric_limits<int>::max(),
96 uint32_t wakeMask = -1);
100 /** Underlying implementation of futexWait and futexWaitUntil.
101 * At most one of absSystemTime and absSteadyTime should be non-null.
102 * Timeouts are separated into separate parameters to allow the
103 * implementations to be elsewhere without templating on the clock
104 * type, which is otherwise complicated by the fact that steady_clock
105 * is the same as system_clock on some platforms. */
106 FutexResult futexWaitImpl(
108 std::chrono::time_point<std::chrono::system_clock>* absSystemTime,
109 std::chrono::time_point<std::chrono::steady_clock>* absSteadyTime,
113 /** A std::atomic subclass that can be used to force Futex to emulate
114 * the underlying futex() syscall. This is primarily useful to test or
115 * benchmark the emulated implementation on systems that don't need it. */
116 template <typename T>
117 struct EmulatedFutexAtomic : public std::atomic<T> {
118 EmulatedFutexAtomic() noexcept = default;
119 constexpr /* implicit */ EmulatedFutexAtomic(T init) noexcept
120 : std::atomic<T>(init) {}
121 EmulatedFutexAtomic(const EmulatedFutexAtomic& rhs) = delete;
124 /* Available specializations, with definitions elsewhere */
127 int Futex<std::atomic>::futexWake(int count, uint32_t wakeMask);
130 FutexResult Futex<std::atomic>::futexWaitImpl(
132 std::chrono::time_point<std::chrono::system_clock>* absSystemTime,
133 std::chrono::time_point<std::chrono::steady_clock>* absSteadyTime,
137 int Futex<EmulatedFutexAtomic>::futexWake(int count, uint32_t wakeMask);
140 FutexResult Futex<EmulatedFutexAtomic>::futexWaitImpl(
142 std::chrono::time_point<std::chrono::system_clock>* absSystemTime,
143 std::chrono::time_point<std::chrono::steady_clock>* absSteadyTime,