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, ATOMIC_ISIZE_INIT};
  20 use sync::{StaticMutex, MUTEX_INIT};
  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 {
  48     mutex: MUTEX_INIT,
  49     cnt: ATOMIC_ISIZE_INIT,
  50     lock_cnt: ATOMIC_ISIZE_INIT,
  51 };
  52
  53 impl Once {
  54     /// Performs an initialization routine once and only once. The given closure
  55     /// will be executed if this is the first time `call_once` has been called,
  56     /// and otherwise the routine will *not* be invoked.
  57     ///
  58     /// This method will block the calling thread if another initialization
  59     /// routine is currently running.
  60     ///
  61     /// When this function returns, it is guaranteed that some initialization
  62     /// has run and completed (it may not be the closure specified). It is also
  63     /// guaranteed that any memory writes performed by the executed closure can
  64     /// be reliably observed by other threads at this point (there is a
  65     /// happens-before relation between the closure and code executing after the
  66     /// return).
  67     #[stable(feature = "rust1", since = "1.0.0")]
  68     pub fn call_once<F>(&'static self, f: F) where F: FnOnce() {
  69         // Optimize common path: load is much cheaper than fetch_add.
  70         if self.cnt.load(Ordering::SeqCst) < 0 {
  71             return
  72         }
  73
  74         // Implementation-wise, this would seem like a fairly trivial primitive.
  75         // The stickler part is where our mutexes currently require an
  76         // allocation, and usage of a `Once` shouldn't leak this allocation.
  77         //
  78         // This means that there must be a deterministic destroyer of the mutex
  79         // contained within (because it's not needed after the initialization
  80         // has run).
  81         //
  82         // The general scheme here is to gate all future threads once
  83         // initialization has completed with a "very negative" count, and to
  84         // allow through threads to lock the mutex if they see a non negative
  85         // count. For all threads grabbing the mutex, exactly one of them should
  86         // be responsible for unlocking the mutex, and this should only be done
  87         // once everyone else is done with the mutex.
  88         //
  89         // This atomicity is achieved by swapping a very negative value into the
  90         // shared count when the initialization routine has completed. This will
  91         // read the number of threads which will at some point attempt to
  92         // acquire the mutex. This count is then squirreled away in a separate
  93         // variable, and the last person on the way out of the mutex is then
  94         // responsible for destroying the mutex.
  95         //
  96         // It is crucial that the negative value is swapped in *after* the
  97         // initialization routine has completed because otherwise new threads
  98         // calling `call_once` will return immediately before the initialization
  99         // has completed.
 100
 101         let prev = self.cnt.fetch_add(1, Ordering::SeqCst);
 102         if prev < 0 {
 103             // Make sure we never overflow, we'll never have isize::MIN
 104             // simultaneous calls to `call_once` to make this value go back to 0
 105             self.cnt.store(isize::MIN, Ordering::SeqCst);
 106             return
 107         }
 108
 109         // If the count is negative, then someone else finished the job,
 110         // otherwise we run the job and record how many people will try to grab
 111         // this lock
 112         let guard = self.mutex.lock();
 113         if self.cnt.load(Ordering::SeqCst) > 0 {
 114             f();
 115             let prev = self.cnt.swap(isize::MIN, Ordering::SeqCst);
 116             self.lock_cnt.store(prev, Ordering::SeqCst);
 117         }
 118         drop(guard);
 119
 120         // Last one out cleans up after everyone else, no leaks!
 121         if self.lock_cnt.fetch_add(-1, Ordering::SeqCst) == 1 {
 122             unsafe { self.mutex.destroy() }
 123         }
 124     }
 125 }
 126
 127 #[cfg(test)]
 128 mod tests {
 129     use prelude::v1::*;
 130
 131     use thread;
 132     use super::{ONCE_INIT, Once};
 133     use sync::mpsc::channel;
 134
 135     #[test]
 136     fn smoke_once() {
 137         static O: Once = ONCE_INIT;
 138         let mut a = 0;
 139         O.call_once(|| a += 1);
 140         assert_eq!(a, 1);
 141         O.call_once(|| a += 1);
 142         assert_eq!(a, 1);
 143     }
 144
 145     #[test]
 146     fn stampede_once() {
 147         static O: Once = ONCE_INIT;
 148         static mut run: bool = false;
 149
 150         let (tx, rx) = channel();
 151         for _ in 0..10 {
 152             let tx = tx.clone();
 153             thread::spawn(move|| {
 154                 for _ in 0..4 { thread::yield_now() }
 155                 unsafe {
 156                     O.call_once(|| {
 157                         assert!(!run);
 158                         run = true;
 159                     });
 160                     assert!(run);
 161                 }
 162                 tx.send(()).unwrap();
 163             });
 164         }
 165
 166         unsafe {
 167             O.call_once(|| {
 168                 assert!(!run);
 169                 run = true;
 170             });
 171             assert!(run);
 172         }
 173
 174         for _ in 0..10 {
 175             rx.recv().unwrap();
 176         }
 177     }
 178 }