2 * Copyright 2016 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 // We launder the clock type via a std::chrono::duration so that we
84 // can compile both the true and false branch. Tricky case is when
85 // steady_clock has a higher precision than system_clock (Xcode 6,
86 // for example), for which time_point<system_clock> construction
87 // refuses to do an implicit duration conversion. (duration is
88 // happy to implicitly convert its denominator causing overflow, but
89 // refuses conversion that might cause truncation.) We use explicit
90 // duration_cast to work around this. Truncation does not actually
91 // occur (unless Duration != Clock::duration) because the missing
92 // implicit conversion is in the untaken branch.
93 Duration absTimeDuration = absTime.time_since_epoch();
94 if (std::is_same<Clock, system_clock>::value) {
95 time_point<system_clock> absSystemTime(
96 duration_cast<system_clock::duration>(absTimeDuration));
97 return futexWaitImpl(expected, &absSystemTime, nullptr, waitMask);
99 time_point<steady_clock> absSteadyTime(
100 duration_cast<steady_clock::duration>(absTimeDuration));
101 return futexWaitImpl(expected, nullptr, &absSteadyTime, waitMask);
105 /** Wakens up to count waiters where (waitMask & wakeMask) !=
106 * 0, returning the number of awoken threads, or -1 if an error
107 * occurred. Note that when constructing a concurrency primitive
108 * that can guard its own destruction, it is likely that you will
109 * want to ignore EINVAL here (as well as making sure that you
110 * never touch the object after performing the memory store that
111 * is the linearization point for unlock or control handoff).
112 * See https://sourceware.org/bugzilla/show_bug.cgi?id=13690 */
113 int futexWake(int count = std::numeric_limits<int>::max(),
114 uint32_t wakeMask = -1);
118 /** Underlying implementation of futexWait and futexWaitUntil.
119 * At most one of absSystemTime and absSteadyTime should be non-null.
120 * Timeouts are separated into separate parameters to allow the
121 * implementations to be elsewhere without templating on the clock
122 * type, which is otherwise complicated by the fact that steady_clock
123 * is the same as system_clock on some platforms. */
124 FutexResult futexWaitImpl(
126 std::chrono::time_point<std::chrono::system_clock>* absSystemTime,
127 std::chrono::time_point<std::chrono::steady_clock>* absSteadyTime,
131 /** A std::atomic subclass that can be used to force Futex to emulate
132 * the underlying futex() syscall. This is primarily useful to test or
133 * benchmark the emulated implementation on systems that don't need it. */
134 template <typename T>
135 struct EmulatedFutexAtomic : public std::atomic<T> {
136 EmulatedFutexAtomic() noexcept = default;
137 constexpr /* implicit */ EmulatedFutexAtomic(T init) noexcept
138 : std::atomic<T>(init) {}
139 // It doesn't copy or move
140 EmulatedFutexAtomic(EmulatedFutexAtomic&& rhs) = delete;
143 /* Available specializations, with definitions elsewhere */
146 int Futex<std::atomic>::futexWake(int count, uint32_t wakeMask);
149 FutexResult Futex<std::atomic>::futexWaitImpl(
151 std::chrono::time_point<std::chrono::system_clock>* absSystemTime,
152 std::chrono::time_point<std::chrono::steady_clock>* absSteadyTime,
156 int Futex<EmulatedFutexAtomic>::futexWake(int count, uint32_t wakeMask);
159 FutexResult Futex<EmulatedFutexAtomic>::futexWaitImpl(
161 std::chrono::time_point<std::chrono::system_clock>* absSystemTime,
162 std::chrono::time_point<std::chrono::steady_clock>* absSteadyTime,