library/std/src/sys/itron/condvar.rs

   1 //! POSIX conditional variable implementation based on user-space wait queues.
   2 use super::{abi, error::expect_success_aborting, spin::SpinMutex, task, time::with_tmos_strong};
   3 use crate::{mem::replace, ptr::NonNull, sys::locks::Mutex, time::Duration};
   4
   5 // The implementation is inspired by the queue-based implementation shown in
   6 // Andrew D. Birrell's paper "Implementing Condition Variables with Semaphores"
   7
   8 pub struct Condvar {
   9     waiters: SpinMutex<waiter_queue::WaiterQueue>,
  10 }
  11
  12 unsafe impl Send for Condvar {}
  13 unsafe impl Sync for Condvar {}
  14
  15 pub type MovableCondvar = Condvar;
  16
  17 impl Condvar {
  18     #[inline]
  19     pub const fn new() -> Condvar {
  20         Condvar { waiters: SpinMutex::new(waiter_queue::WaiterQueue::new()) }
  21     }
  22
  23     #[inline]
  24     pub unsafe fn init(&mut self) {}
  25
  26     pub unsafe fn notify_one(&self) {
  27         self.waiters.with_locked(|waiters| {
  28             if let Some(task) = waiters.pop_front() {
  29                 // Unpark the task
  30                 match unsafe { abi::wup_tsk(task) } {
  31                     // The task already has a token.
  32                     abi::E_QOVR => {}
  33                     // Can't undo the effect; abort the program on failure
  34                     er => {
  35                         expect_success_aborting(er, &"wup_tsk");
  36                     }
  37                 }
  38             }
  39         });
  40     }
  41
  42     pub unsafe fn notify_all(&self) {
  43         self.waiters.with_locked(|waiters| {
  44             while let Some(task) = waiters.pop_front() {
  45                 // Unpark the task
  46                 match unsafe { abi::wup_tsk(task) } {
  47                     // The task already has a token.
  48                     abi::E_QOVR => {}
  49                     // Can't undo the effect; abort the program on failure
  50                     er => {
  51                         expect_success_aborting(er, &"wup_tsk");
  52                     }
  53                 }
  54             }
  55         });
  56     }
  57
  58     pub unsafe fn wait(&self, mutex: &Mutex) {
  59         // Construct `Waiter`.
  60         let mut waiter = waiter_queue::Waiter::new();
  61         let waiter = NonNull::from(&mut waiter);
  62
  63         self.waiters.with_locked(|waiters| unsafe {
  64             waiters.insert(waiter);
  65         });
  66
  67         unsafe { mutex.unlock() };
  68
  69         // Wait until `waiter` is removed from the queue
  70         loop {
  71             // Park the current task
  72             expect_success_aborting(unsafe { abi::slp_tsk() }, &"slp_tsk");
  73
  74             if !self.waiters.with_locked(|waiters| unsafe { waiters.is_queued(waiter) }) {
  75                 break;
  76             }
  77         }
  78
  79         unsafe { mutex.lock() };
  80     }
  81
  82     pub unsafe fn wait_timeout(&self, mutex: &Mutex, dur: Duration) -> bool {
  83         // Construct and pin `Waiter`
  84         let mut waiter = waiter_queue::Waiter::new();
  85         let waiter = NonNull::from(&mut waiter);
  86
  87         self.waiters.with_locked(|waiters| unsafe {
  88             waiters.insert(waiter);
  89         });
  90
  91         unsafe { mutex.unlock() };
  92
  93         // Park the current task and do not wake up until the timeout elapses
  94         // or the task gets woken up by `notify_*`
  95         match with_tmos_strong(dur, |tmo| {
  96             let er = unsafe { abi::tslp_tsk(tmo) };
  97             if er == 0 {
  98                 // We were unparked. Are we really dequeued?
  99                 if self.waiters.with_locked(|waiters| unsafe { waiters.is_queued(waiter) }) {
 100                     // No we are not. Continue waiting.
 101                     return abi::E_TMOUT;
 102                 }
 103             }
 104             er
 105         }) {
 106             abi::E_TMOUT => {}
 107             er => {
 108                 expect_success_aborting(er, &"tslp_tsk");
 109             }
 110         }
 111
 112         // Remove `waiter` from `self.waiters`. If `waiter` is still in
 113         // `waiters`, it means we woke up because of a timeout. Otherwise,
 114         // we woke up because of `notify_*`.
 115         let success = self.waiters.with_locked(|waiters| unsafe { !waiters.remove(waiter) });
 116
 117         unsafe { mutex.lock() };
 118         success
 119     }
 120 }
 121
 122 mod waiter_queue {
 123     use super::*;
 124
 125     pub struct WaiterQueue {
 126         head: Option<ListHead>,
 127     }
 128
 129     #[derive(Copy, Clone)]
 130     struct ListHead {
 131         first: NonNull<Waiter>,
 132         last: NonNull<Waiter>,
 133     }
 134
 135     unsafe impl Send for ListHead {}
 136     unsafe impl Sync for ListHead {}
 137
 138     pub struct Waiter {
 139         // These fields are only accessed through `&[mut] WaiterQueue`.
 140         /// The waiting task's ID. Will be zeroed when the task is woken up
 141         /// and removed from a queue.
 142         task: abi::ID,
 143         priority: abi::PRI,
 144         prev: Option<NonNull<Waiter>>,
 145         next: Option<NonNull<Waiter>>,
 146     }
 147
 148     unsafe impl Send for Waiter {}
 149     unsafe impl Sync for Waiter {}
 150
 151     impl Waiter {
 152         #[inline]
 153         pub fn new() -> Self {
 154             let task = task::current_task_id();
 155             let priority = task::task_priority(abi::TSK_SELF);
 156
 157             // Zeroness of `Waiter::task` indicates whether the `Waiter` is
 158             // linked to a queue or not. This invariant is important for
 159             // the correctness.
 160             debug_assert_ne!(task, 0);
 161
 162             Self { task, priority, prev: None, next: None }
 163         }
 164     }
 165
 166     impl WaiterQueue {
 167         #[inline]
 168         pub const fn new() -> Self {
 169             Self { head: None }
 170         }
 171
 172         /// # Safety
 173         ///
 174         ///  - The caller must own `*waiter_ptr`. The caller will lose the
 175         ///    ownership until `*waiter_ptr` is removed from `self`.
 176         ///
 177         ///  - `*waiter_ptr` must be valid until it's removed from the queue.
 178         ///
 179         ///  - `*waiter_ptr` must not have been previously inserted to a `WaiterQueue`.
 180         ///
 181         pub unsafe fn insert(&mut self, mut waiter_ptr: NonNull<Waiter>) {
 182             unsafe {
 183                 let waiter = waiter_ptr.as_mut();
 184
 185                 debug_assert!(waiter.prev.is_none());
 186                 debug_assert!(waiter.next.is_none());
 187
 188                 if let Some(head) = &mut self.head {
 189                     // Find the insertion position and insert `waiter`
 190                     let insert_after = {
 191                         let mut cursor = head.last;
 192                         loop {
 193                             if waiter.priority >= cursor.as_ref().priority {
 194                                 // `cursor` and all previous waiters have the same or higher
 195                                 // priority than `current_task_priority`. Insert the new
 196                                 // waiter right after `cursor`.
 197                                 break Some(cursor);
 198                             }
 199                             cursor = if let Some(prev) = cursor.as_ref().prev {
 200                                 prev
 201                             } else {
 202                                 break None;
 203                             };
 204                         }
 205                     };
 206
 207                     if let Some(mut insert_after) = insert_after {
 208                         // Insert `waiter` after `insert_after`
 209                         let insert_before = insert_after.as_ref().next;
 210
 211                         waiter.prev = Some(insert_after);
 212                         insert_after.as_mut().next = Some(waiter_ptr);
 213
 214                         waiter.next = insert_before;
 215                         if let Some(mut insert_before) = insert_before {
 216                             insert_before.as_mut().prev = Some(waiter_ptr);
 217                         } else {
 218                             head.last = waiter_ptr;
 219                         }
 220                     } else {
 221                         // Insert `waiter` to the front
 222                         waiter.next = Some(head.first);
 223                         head.first.as_mut().prev = Some(waiter_ptr);
 224                         head.first = waiter_ptr;
 225                     }
 226                 } else {
 227                     // `waiter` is the only element
 228                     self.head = Some(ListHead { first: waiter_ptr, last: waiter_ptr });
 229                 }
 230             }
 231         }
 232
 233         /// Given a `Waiter` that was previously inserted to `self`, remove
 234         /// it from `self` if it's still there.
 235         #[inline]
 236         pub unsafe fn remove(&mut self, mut waiter_ptr: NonNull<Waiter>) -> bool {
 237             unsafe {
 238                 let waiter = waiter_ptr.as_mut();
 239                 if waiter.task != 0 {
 240                     let head = self.head.as_mut().unwrap();
 241
 242                     match (waiter.prev, waiter.next) {
 243                         (Some(mut prev), Some(mut next)) => {
 244                             prev.as_mut().next = Some(next);
 245                             next.as_mut().prev = Some(prev);
 246                         }
 247                         (None, Some(mut next)) => {
 248                             head.first = next;
 249                             next.as_mut().prev = None;
 250                         }
 251                         (Some(mut prev), None) => {
 252                             prev.as_mut().next = None;
 253                             head.last = prev;
 254                         }
 255                         (None, None) => {
 256                             self.head = None;
 257                         }
 258                     }
 259
 260                     waiter.task = 0;
 261
 262                     true
 263                 } else {
 264                     false
 265                 }
 266             }
 267         }
 268
 269         /// Given a `Waiter` that was previously inserted to `self`, return a
 270         /// flag indicating whether it's still in `self`.
 271         #[inline]
 272         pub unsafe fn is_queued(&self, waiter: NonNull<Waiter>) -> bool {
 273             unsafe { waiter.as_ref().task != 0 }
 274         }
 275
 276         #[inline]
 277         pub fn pop_front(&mut self) -> Option<abi::ID> {
 278             unsafe {
 279                 let head = self.head.as_mut()?;
 280                 let waiter = head.first.as_mut();
 281
 282                 // Get the ID
 283                 let id = replace(&mut waiter.task, 0);
 284
 285                 // Unlink the waiter
 286                 if let Some(mut next) = waiter.next {
 287                     head.first = next;
 288                     next.as_mut().prev = None;
 289                 } else {
 290                     self.head = None;
 291                 }
 292
 293                 Some(id)
 294             }
 295         }
 296     }
 297 }