src/libstd/sync/once.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! A "once initialization" primitive
  12 //!
  13 //! This primitive is meant to be used to run one-time initialization. An
  14 //! example use case would be for initializing an FFI library.
  15
  16 use prelude::v1::*;
  17
  18 use isize;
  19 use sync::atomic::{AtomicIsize, Ordering};
  20 use sync::StaticMutex;
  21
  22 /// A synchronization primitive which can be used to run a one-time global
  23 /// initialization. Useful for one-time initialization for FFI or related
  24 /// functionality. This type can only be constructed with the `ONCE_INIT`
  25 /// value.
  26 ///
  27 /// # Examples
  28 ///
  29 /// ```
  30 /// use std::sync::{Once, ONCE_INIT};
  31 ///
  32 /// static START: Once = ONCE_INIT;
  33 ///
  34 /// START.call_once(|| {
  35 ///     // run initialization here
  36 /// });
  37 /// ```
  38 #[stable(feature = "rust1", since = "1.0.0")]
  39 pub struct Once {
  40     mutex: StaticMutex,
  41     cnt: AtomicIsize,
  42     lock_cnt: AtomicIsize,
  43 }
  44
  45 /// Initialization value for static `Once` values.
  46 #[stable(feature = "rust1", since = "1.0.0")]
  47 pub const ONCE_INIT: Once = Once::new();
  48
  49 impl Once {
  50     /// Creates a new `Once` value.
  51     #[stable(feature = "once_new", since = "1.2.0")]
  52     pub const fn new() -> Once {
  53         Once {
  54             mutex: StaticMutex::new(),
  55             cnt: AtomicIsize::new(0),
  56             lock_cnt: AtomicIsize::new(0),
  57         }
  58     }
  59
  60     /// Performs an initialization routine once and only once. The given closure
  61     /// will be executed if this is the first time `call_once` has been called,
  62     /// and otherwise the routine will *not* be invoked.
  63     ///
  64     /// This method will block the calling thread if another initialization
  65     /// routine is currently running.
  66     ///
  67     /// When this function returns, it is guaranteed that some initialization
  68     /// has run and completed (it may not be the closure specified). It is also
  69     /// guaranteed that any memory writes performed by the executed closure can
  70     /// be reliably observed by other threads at this point (there is a
  71     /// happens-before relation between the closure and code executing after the
  72     /// return).
  73     #[stable(feature = "rust1", since = "1.0.0")]
  74     pub fn call_once<F>(&'static self, f: F) where F: FnOnce() {
  75         // Optimize common path: load is much cheaper than fetch_add.
  76         if self.cnt.load(Ordering::SeqCst) < 0 {
  77             return
  78         }
  79
  80         // Implementation-wise, this would seem like a fairly trivial primitive.
  81         // The stickler part is where our mutexes currently require an
  82         // allocation, and usage of a `Once` shouldn't leak this allocation.
  83         //
  84         // This means that there must be a deterministic destroyer of the mutex
  85         // contained within (because it's not needed after the initialization
  86         // has run).
  87         //
  88         // The general scheme here is to gate all future threads once
  89         // initialization has completed with a "very negative" count, and to
  90         // allow through threads to lock the mutex if they see a non negative
  91         // count. For all threads grabbing the mutex, exactly one of them should
  92         // be responsible for unlocking the mutex, and this should only be done
  93         // once everyone else is done with the mutex.
  94         //
  95         // This atomicity is achieved by swapping a very negative value into the
  96         // shared count when the initialization routine has completed. This will
  97         // read the number of threads which will at some point attempt to
  98         // acquire the mutex. This count is then squirreled away in a separate
  99         // variable, and the last person on the way out of the mutex is then
 100         // responsible for destroying the mutex.
 101         //
 102         // It is crucial that the negative value is swapped in *after* the
 103         // initialization routine has completed because otherwise new threads
 104         // calling `call_once` will return immediately before the initialization
 105         // has completed.
 106
 107         let prev = self.cnt.fetch_add(1, Ordering::SeqCst);
 108         if prev < 0 {
 109             // Make sure we never overflow, we'll never have isize::MIN
 110             // simultaneous calls to `call_once` to make this value go back to 0
 111             self.cnt.store(isize::MIN, Ordering::SeqCst);
 112             return
 113         }
 114
 115         // If the count is negative, then someone else finished the job,
 116         // otherwise we run the job and record how many people will try to grab
 117         // this lock
 118         let guard = self.mutex.lock();
 119         if self.cnt.load(Ordering::SeqCst) > 0 {
 120             f();
 121             let prev = self.cnt.swap(isize::MIN, Ordering::SeqCst);
 122             self.lock_cnt.store(prev, Ordering::SeqCst);
 123         }
 124         drop(guard);
 125
 126         // Last one out cleans up after everyone else, no leaks!
 127         if self.lock_cnt.fetch_add(-1, Ordering::SeqCst) == 1 {
 128             unsafe { self.mutex.destroy() }
 129         }
 130     }
 131 }
 132
 133 #[cfg(test)]
 134 mod tests {
 135     use prelude::v1::*;
 136
 137     use thread;
 138     use super::Once;
 139     use sync::mpsc::channel;
 140
 141     #[test]
 142     fn smoke_once() {
 143         static O: Once = Once::new();
 144         let mut a = 0;
 145         O.call_once(|| a += 1);
 146         assert_eq!(a, 1);
 147         O.call_once(|| a += 1);
 148         assert_eq!(a, 1);
 149     }
 150
 151     #[test]
 152     fn stampede_once() {
 153         static O: Once = Once::new();
 154         static mut run: bool = false;
 155
 156         let (tx, rx) = channel();
 157         for _ in 0..10 {
 158             let tx = tx.clone();
 159             thread::spawn(move|| {
 160                 for _ in 0..4 { thread::yield_now() }
 161                 unsafe {
 162                     O.call_once(|| {
 163                         assert!(!run);
 164                         run = true;
 165                     });
 166                     assert!(run);
 167                 }
 168                 tx.send(()).unwrap();
 169             });
 170         }
 171
 172         unsafe {
 173             O.call_once(|| {
 174                 assert!(!run);
 175                 run = true;
 176             });
 177             assert!(run);
 178         }
 179
 180         for _ in 0..10 {
 181             rx.recv().unwrap();
 182         }
 183     }
 184 }