      1
      2
      3
      4
      5
      6
      7
      8
      9
     10
     11
     12
     13
     14
     15
     16
     17
     18
     19
     20
     21
     22
     23
     24
     25
     26
     27
     28
     29
     30
     31
     32
     33
     34
     35
     36
     37
     38
     39
     40
     41
     42
     43
     44
     45
     46
     47
     48
     49
     50
     51
     52
     53
     54
     55
     56
     57
     58
     59
     60
     61
     62
     63
     64
     65
     66
     67
     68
     69
     70
     71
     72
     73
     74
     75
     76
     77
     78
     79
     80
     81
     82
     83
     84
     85
     86
     87
     88
     89
     90
     91
     92
     93
     94
     95
     96
     97
     98
     99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156
    157
    158
    159
    160
    161
    162
    163
    164
    165
    166
    167
    168
    169
    170
    171
    172
    173
    174
    175
    176
    177
    178
    179
    180
    181
    182
    183
    184
    185
    186
    187
    188
    189
    190
    191
    192
    193
    194
    195
    196
    197
    198
    199
    200
    201
    202
    203
    204
    205
    206
    207
    208
    209
    210
    211
    212
    213
    214
    215
    216
    217
    218
    219
    220
    221
    222
    223
    224
    225
    226
    227
    228
    229
    230
    231
    232
    233
    234
    235
    236
    237
    238
    239
    240
    241
    242
    243
    244
    245
    246
    247
    248
    249
    250
    251
    252
    253
    254
    255
    256
    257

    //! Parks the runtime.
    //!
    //! A combination of the various resource driver park handles.

    use crate::loom::sync::atomic::AtomicUsize;
    use crate::loom::sync::{Arc, Condvar, Mutex};
    use crate::loom::thread;
    use crate::park::{Park, Unpark};
    use crate::runtime::driver::Driver;
    use crate::util::TryLock;

    use std::sync::atomic::Ordering::SeqCst;
    use std::time::Duration;

    pub(crate) struct Parker {
        inner: Arc<Inner>,
    }

    pub(crate) struct Unparker {
        inner: Arc<Inner>,
    }

    struct Inner {
        /// Avoids entering the park if possible
        state: AtomicUsize,

        /// Used to coordinate access to the driver / condvar
        mutex: Mutex<()>,

        /// Condvar to block on if the driver is unavailable.
        condvar: Condvar,

        /// Resource (I/O, time, ...) driver
        shared: Arc<Shared>,
    }

    const EMPTY: usize = 0;
    const PARKED_CONDVAR: usize = 1;
    const PARKED_DRIVER: usize = 2;
    const NOTIFIED: usize = 3;

    /// Shared across multiple Parker handles
    struct Shared {
        /// Shared driver. Only one thread at a time can use this
        driver: TryLock<Driver>,

        /// Unpark handle
        handle: <Driver as Park>::Unpark,
    }

    impl Parker {
        pub(crate) fn new(driver: Driver) -> Parker {
            let handle = driver.unpark();

            Parker {
                inner: Arc::new(Inner {
                    state: AtomicUsize::new(EMPTY),
                    mutex: Mutex::new(()),
                    condvar: Condvar::new(),
                    shared: Arc::new(Shared {
                        driver: TryLock::new(driver),
                        handle,
                    }),
                }),
            }
        }
    }

    impl Clone for Parker {
        fn clone(&self) -> Parker {
            Parker {
                inner: Arc::new(Inner {
                    state: AtomicUsize::new(EMPTY),
                    mutex: Mutex::new(()),
                    condvar: Condvar::new(),
                    shared: self.inner.shared.clone(),
                }),
            }
        }
    }

    impl Park for Parker {
        type Unpark = Unparker;
        type Error = ();

        fn unpark(&self) -> Unparker {
            Unparker {
                inner: self.inner.clone(),
            }
        }

        fn park(&mut self) -> Result<(), Self::Error> {
            self.inner.park();
            Ok(())
        }

        fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error> {
            // Only parking with zero is supported...
            assert_eq!(duration, Duration::from_millis(0));

            if let Some(mut driver) = self.inner.shared.driver.try_lock() {
                driver.park_timeout(duration).map_err(|_| ())
            } else {
                Ok(())
            }
        }

        fn shutdown(&mut self) {
            self.inner.shutdown();
        }
    }

    impl Unpark for Unparker {
        fn unpark(&self) {
            self.inner.unpark();
        }
    }

    impl Inner {
        /// Parks the current thread for at most `dur`.
        fn park(&self) {
            for _ in 0..3 {
                // If we were previously notified then we consume this notification and
                // return quickly.
                if self
                    .state
                    .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
                    .is_ok()
                {
                    return;
                }

                thread::yield_now();
            }

            if let Some(mut driver) = self.shared.driver.try_lock() {
                self.park_driver(&mut driver);
            } else {
                self.park_condvar();
            }
        }

        fn park_condvar(&self) {
            // Otherwise we need to coordinate going to sleep
            let mut m = self.mutex.lock();

            match self
                .state
                .compare_exchange(EMPTY, PARKED_CONDVAR, SeqCst, SeqCst)
            {
                Ok(_) => {}
                Err(NOTIFIED) => {
                    // We must read here, even though we know it will be `NOTIFIED`.
                    // This is because `unpark` may have been called again since we read
                    // `NOTIFIED` in the `compare_exchange` above. We must perform an
                    // acquire operation that synchronizes with that `unpark` to observe
                    // any writes it made before the call to unpark. To do that we must
                    // read from the write it made to `state`.
                    let old = self.state.swap(EMPTY, SeqCst);
                    debug_assert_eq!(old, NOTIFIED, "park state changed unexpectedly");

                    return;
                }
                Err(actual) => panic!("inconsistent park state; actual = {}", actual),
            }

            loop {
                m = self.condvar.wait(m).unwrap();

                if self
                    .state
                    .compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst)
                    .is_ok()
                {
                    // got a notification
                    return;
                }

                // spurious wakeup, go back to sleep
            }
        }

        fn park_driver(&self, driver: &mut Driver) {
            match self
                .state
                .compare_exchange(EMPTY, PARKED_DRIVER, SeqCst, SeqCst)
            {
                Ok(_) => {}
                Err(NOTIFIED) => {
                    // We must read here, even though we know it will be `NOTIFIED`.
                    // This is because `unpark` may have been called again since we read
                    // `NOTIFIED` in the `compare_exchange` above. We must perform an
                    // acquire operation that synchronizes with that `unpark` to observe
                    // any writes it made before the call to unpark. To do that we must
                    // read from the write it made to `state`.
                    let old = self.state.swap(EMPTY, SeqCst);
                    debug_assert_eq!(old, NOTIFIED, "park state changed unexpectedly");

                    return;
                }
                Err(actual) => panic!("inconsistent park state; actual = {}", actual),
            }

            // TODO: don't unwrap
            driver.park().unwrap();

            match self.state.swap(EMPTY, SeqCst) {
                NOTIFIED => {}      // got a notification, hurray!
                PARKED_DRIVER => {} // no notification, alas
                n => panic!("inconsistent park_timeout state: {}", n),
            }
        }

        fn unpark(&self) {
            // To ensure the unparked thread will observe any writes we made before
            // this call, we must perform a release operation that `park` can
            // synchronize with. To do that we must write `NOTIFIED` even if `state`
            // is already `NOTIFIED`. That is why this must be a swap rather than a
            // compare-and-swap that returns if it reads `NOTIFIED` on failure.
            match self.state.swap(NOTIFIED, SeqCst) {
                EMPTY => {}    // no one was waiting
                NOTIFIED => {} // already unparked
                PARKED_CONDVAR => self.unpark_condvar(),
                PARKED_DRIVER => self.unpark_driver(),
                actual => panic!("inconsistent state in unpark; actual = {}", actual),
            }
        }

        fn unpark_condvar(&self) {
            // There is a period between when the parked thread sets `state` to
            // `PARKED` (or last checked `state` in the case of a spurious wake
            // up) and when it actually waits on `cvar`. If we were to notify
            // during this period it would be ignored and then when the parked
            // thread went to sleep it would never wake up. Fortunately, it has
            // `lock` locked at this stage so we can acquire `lock` to wait until
            // it is ready to receive the notification.
            //
            // Releasing `lock` before the call to `notify_one` means that when the
            // parked thread wakes it doesn't get woken only to have to wait for us
            // to release `lock`.
            drop(self.mutex.lock());

            self.condvar.notify_one()
        }

        fn unpark_driver(&self) {
            self.shared.handle.unpark();
        }

        fn shutdown(&self) {
            if let Some(mut driver) = self.shared.driver.try_lock() {
                driver.shutdown();
            }

            self.condvar.notify_all();
        }
    }