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};
5 // The implementation is inspired by the queue-based implementation shown in
6 // Andrew D. Birrell's paper "Implementing Condition Variables with Semaphores"
9 waiters: SpinMutex<waiter_queue::WaiterQueue>,
12 unsafe impl Send for Condvar {}
13 unsafe impl Sync for Condvar {}
17 pub const fn new() -> Condvar {
18 Condvar { waiters: SpinMutex::new(waiter_queue::WaiterQueue::new()) }
21 pub fn notify_one(&self) {
22 self.waiters.with_locked(|waiters| {
23 if let Some(task) = waiters.pop_front() {
25 match unsafe { abi::wup_tsk(task) } {
26 // The task already has a token.
28 // Can't undo the effect; abort the program on failure
30 expect_success_aborting(er, &"wup_tsk");
37 pub fn notify_all(&self) {
38 self.waiters.with_locked(|waiters| {
39 while let Some(task) = waiters.pop_front() {
41 match unsafe { abi::wup_tsk(task) } {
42 // The task already has a token.
44 // Can't undo the effect; abort the program on failure
46 expect_success_aborting(er, &"wup_tsk");
53 pub unsafe fn wait(&self, mutex: &Mutex) {
54 // Construct `Waiter`.
55 let mut waiter = waiter_queue::Waiter::new();
56 let waiter = NonNull::from(&mut waiter);
58 self.waiters.with_locked(|waiters| unsafe {
59 waiters.insert(waiter);
62 unsafe { mutex.unlock() };
64 // Wait until `waiter` is removed from the queue
66 // Park the current task
67 expect_success_aborting(unsafe { abi::slp_tsk() }, &"slp_tsk");
69 if !self.waiters.with_locked(|waiters| unsafe { waiters.is_queued(waiter) }) {
74 unsafe { mutex.lock() };
77 pub unsafe fn wait_timeout(&self, mutex: &Mutex, dur: Duration) -> bool {
78 // Construct and pin `Waiter`
79 let mut waiter = waiter_queue::Waiter::new();
80 let waiter = NonNull::from(&mut waiter);
82 self.waiters.with_locked(|waiters| unsafe {
83 waiters.insert(waiter);
86 unsafe { mutex.unlock() };
88 // Park the current task and do not wake up until the timeout elapses
89 // or the task gets woken up by `notify_*`
90 match with_tmos_strong(dur, |tmo| {
91 let er = unsafe { abi::tslp_tsk(tmo) };
93 // We were unparked. Are we really dequeued?
94 if self.waiters.with_locked(|waiters| unsafe { waiters.is_queued(waiter) }) {
95 // No we are not. Continue waiting.
103 expect_success_aborting(er, &"tslp_tsk");
107 // Remove `waiter` from `self.waiters`. If `waiter` is still in
108 // `waiters`, it means we woke up because of a timeout. Otherwise,
109 // we woke up because of `notify_*`.
110 let success = self.waiters.with_locked(|waiters| unsafe { !waiters.remove(waiter) });
112 unsafe { mutex.lock() };
120 pub struct WaiterQueue {
121 head: Option<ListHead>,
124 #[derive(Copy, Clone)]
126 first: NonNull<Waiter>,
127 last: NonNull<Waiter>,
130 unsafe impl Send for ListHead {}
131 unsafe impl Sync for ListHead {}
134 // These fields are only accessed through `&[mut] WaiterQueue`.
135 /// The waiting task's ID. Will be zeroed when the task is woken up
136 /// and removed from a queue.
139 prev: Option<NonNull<Waiter>>,
140 next: Option<NonNull<Waiter>>,
143 unsafe impl Send for Waiter {}
144 unsafe impl Sync for Waiter {}
148 pub fn new() -> Self {
149 let task = task::current_task_id();
150 let priority = task::task_priority(abi::TSK_SELF);
152 // Zeroness of `Waiter::task` indicates whether the `Waiter` is
153 // linked to a queue or not. This invariant is important for
155 debug_assert_ne!(task, 0);
157 Self { task, priority, prev: None, next: None }
163 pub const fn new() -> Self {
169 /// - The caller must own `*waiter_ptr`. The caller will lose the
170 /// ownership until `*waiter_ptr` is removed from `self`.
172 /// - `*waiter_ptr` must be valid until it's removed from the queue.
174 /// - `*waiter_ptr` must not have been previously inserted to a `WaiterQueue`.
176 pub unsafe fn insert(&mut self, mut waiter_ptr: NonNull<Waiter>) {
178 let waiter = waiter_ptr.as_mut();
180 debug_assert!(waiter.prev.is_none());
181 debug_assert!(waiter.next.is_none());
183 if let Some(head) = &mut self.head {
184 // Find the insertion position and insert `waiter`
186 let mut cursor = head.last;
188 if waiter.priority >= cursor.as_ref().priority {
189 // `cursor` and all previous waiters have the same or higher
190 // priority than `current_task_priority`. Insert the new
191 // waiter right after `cursor`.
194 cursor = if let Some(prev) = cursor.as_ref().prev {
202 if let Some(mut insert_after) = insert_after {
203 // Insert `waiter` after `insert_after`
204 let insert_before = insert_after.as_ref().next;
206 waiter.prev = Some(insert_after);
207 insert_after.as_mut().next = Some(waiter_ptr);
209 waiter.next = insert_before;
210 if let Some(mut insert_before) = insert_before {
211 insert_before.as_mut().prev = Some(waiter_ptr);
213 head.last = waiter_ptr;
216 // Insert `waiter` to the front
217 waiter.next = Some(head.first);
218 head.first.as_mut().prev = Some(waiter_ptr);
219 head.first = waiter_ptr;
222 // `waiter` is the only element
223 self.head = Some(ListHead { first: waiter_ptr, last: waiter_ptr });
228 /// Given a `Waiter` that was previously inserted to `self`, remove
229 /// it from `self` if it's still there.
231 pub unsafe fn remove(&mut self, mut waiter_ptr: NonNull<Waiter>) -> bool {
233 let waiter = waiter_ptr.as_mut();
234 if waiter.task != 0 {
235 let head = self.head.as_mut().unwrap();
237 match (waiter.prev, waiter.next) {
238 (Some(mut prev), Some(mut next)) => {
239 prev.as_mut().next = Some(next);
240 next.as_mut().prev = Some(prev);
242 (None, Some(mut next)) => {
244 next.as_mut().prev = None;
246 (Some(mut prev), None) => {
247 prev.as_mut().next = None;
264 /// Given a `Waiter` that was previously inserted to `self`, return a
265 /// flag indicating whether it's still in `self`.
267 pub unsafe fn is_queued(&self, waiter: NonNull<Waiter>) -> bool {
268 unsafe { waiter.as_ref().task != 0 }
272 pub fn pop_front(&mut self) -> Option<abi::ID> {
274 let head = self.head.as_mut()?;
275 let waiter = head.first.as_mut();
278 let id = replace(&mut waiter.task, 0);
281 if let Some(mut next) = waiter.next {
283 next.as_mut().prev = None;