1//! A `Barrier` that provides `wait_timeout`.
2//!
3//! This implementation mirrors that of the Rust standard library.
4
5use crate::loom::sync::{Condvar, Mutex};
6use std::fmt;
7use std::time::{Duration, Instant};
8
9/// A barrier enables multiple threads to synchronize the beginning
10/// of some computation.
11///
12/// # Examples
13///
14/// ```
15/// use std::sync::{Arc, Barrier};
16/// use std::thread;
17///
18/// let mut handles = Vec::with_capacity(10);
19/// let barrier = Arc::new(Barrier::new(10));
20/// for _ in 0..10 {
21/// let c = Arc::clone(&barrier);
22/// // The same messages will be printed together.
23/// // You will NOT see any interleaving.
24/// handles.push(thread::spawn(move|| {
25/// println!("before wait");
26/// c.wait();
27/// println!("after wait");
28/// }));
29/// }
30/// // Wait for other threads to finish.
31/// for handle in handles {
32/// handle.join().unwrap();
33/// }
34/// ```
35pub(crate) struct Barrier {
36 lock: Mutex<BarrierState>,
37 cvar: Condvar,
38 num_threads: usize,
39}
40
41// The inner state of a double barrier
42struct BarrierState {
43 count: usize,
44 generation_id: usize,
45}
46
47/// A `BarrierWaitResult` is returned by [`Barrier::wait()`] when all threads
48/// in the [`Barrier`] have rendezvoused.
49///
50/// # Examples
51///
52/// ```
53/// use std::sync::Barrier;
54///
55/// let barrier = Barrier::new(1);
56/// let barrier_wait_result = barrier.wait();
57/// ```
58pub(crate) struct BarrierWaitResult(bool);
59
60impl fmt::Debug for Barrier {
61 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
62 f.debug_struct("Barrier").finish_non_exhaustive()
63 }
64}
65
66impl Barrier {
67 /// Creates a new barrier that can block a given number of threads.
68 ///
69 /// A barrier will block `n`-1 threads which call [`wait()`] and then wake
70 /// up all threads at once when the `n`th thread calls [`wait()`].
71 ///
72 /// [`wait()`]: Barrier::wait
73 ///
74 /// # Examples
75 ///
76 /// ```
77 /// use std::sync::Barrier;
78 ///
79 /// let barrier = Barrier::new(10);
80 /// ```
81 #[must_use]
82 pub(crate) fn new(n: usize) -> Barrier {
83 Barrier {
84 lock: Mutex::new(BarrierState {
85 count: 0,
86 generation_id: 0,
87 }),
88 cvar: Condvar::new(),
89 num_threads: n,
90 }
91 }
92
93 /// Blocks the current thread until all threads have rendezvoused here.
94 ///
95 /// Barriers are re-usable after all threads have rendezvoused once, and can
96 /// be used continuously.
97 ///
98 /// A single (arbitrary) thread will receive a [`BarrierWaitResult`] that
99 /// returns `true` from [`BarrierWaitResult::is_leader()`] when returning
100 /// from this function, and all other threads will receive a result that
101 /// will return `false` from [`BarrierWaitResult::is_leader()`].
102 ///
103 /// # Examples
104 ///
105 /// ```
106 /// use std::sync::{Arc, Barrier};
107 /// use std::thread;
108 ///
109 /// let mut handles = Vec::with_capacity(10);
110 /// let barrier = Arc::new(Barrier::new(10));
111 /// for _ in 0..10 {
112 /// let c = Arc::clone(&barrier);
113 /// // The same messages will be printed together.
114 /// // You will NOT see any interleaving.
115 /// handles.push(thread::spawn(move|| {
116 /// println!("before wait");
117 /// c.wait();
118 /// println!("after wait");
119 /// }));
120 /// }
121 /// // Wait for other threads to finish.
122 /// for handle in handles {
123 /// handle.join().unwrap();
124 /// }
125 /// ```
126 pub(crate) fn wait(&self) -> BarrierWaitResult {
127 let mut lock = self.lock.lock();
128 let local_gen = lock.generation_id;
129 lock.count += 1;
130 if lock.count < self.num_threads {
131 // We need a while loop to guard against spurious wakeups.
132 // https://en.wikipedia.org/wiki/Spurious_wakeup
133 while local_gen == lock.generation_id {
134 lock = self.cvar.wait(lock).unwrap();
135 }
136 BarrierWaitResult(false)
137 } else {
138 lock.count = 0;
139 lock.generation_id = lock.generation_id.wrapping_add(1);
140 self.cvar.notify_all();
141 BarrierWaitResult(true)
142 }
143 }
144
145 /// Blocks the current thread until all threads have rendezvoused here for
146 /// at most `timeout` duration.
147 pub(crate) fn wait_timeout(&self, timeout: Duration) -> Option<BarrierWaitResult> {
148 // This implementation mirrors `wait`, but with each blocking operation
149 // replaced by a timeout-amenable alternative.
150
151 let deadline = Instant::now() + timeout;
152
153 // Acquire `self.lock` with at most `timeout` duration.
154 let mut lock = loop {
155 if let Some(guard) = self.lock.try_lock() {
156 break guard;
157 } else if Instant::now() > deadline {
158 return None;
159 } else {
160 std::thread::yield_now();
161 }
162 };
163
164 // Shrink the `timeout` to account for the time taken to acquire `lock`.
165 let timeout = deadline.saturating_duration_since(Instant::now());
166
167 let local_gen = lock.generation_id;
168 lock.count += 1;
169 if lock.count < self.num_threads {
170 // We need a while loop to guard against spurious wakeups.
171 // https://en.wikipedia.org/wiki/Spurious_wakeup
172 while local_gen == lock.generation_id {
173 let (guard, timeout_result) = self.cvar.wait_timeout(lock, timeout).unwrap();
174 lock = guard;
175 if timeout_result.timed_out() {
176 return None;
177 }
178 }
179 Some(BarrierWaitResult(false))
180 } else {
181 lock.count = 0;
182 lock.generation_id = lock.generation_id.wrapping_add(1);
183 self.cvar.notify_all();
184 Some(BarrierWaitResult(true))
185 }
186 }
187}
188
189impl fmt::Debug for BarrierWaitResult {
190 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
191 f.debug_struct("BarrierWaitResult")
192 .field("is_leader", &self.is_leader())
193 .finish()
194 }
195}
196
197impl BarrierWaitResult {
198 /// Returns `true` if this thread is the "leader thread" for the call to
199 /// [`Barrier::wait()`].
200 ///
201 /// Only one thread will have `true` returned from their result, all other
202 /// threads will have `false` returned.
203 ///
204 /// # Examples
205 ///
206 /// ```
207 /// use std::sync::Barrier;
208 ///
209 /// let barrier = Barrier::new(1);
210 /// let barrier_wait_result = barrier.wait();
211 /// println!("{:?}", barrier_wait_result.is_leader());
212 /// ```
213 #[must_use]
214 pub(crate) fn is_leader(&self) -> bool {
215 self.0
216 }
217}
218