1 // SPDX-License-Identifier: GPL-2.0
2
3 //! A condition variable.
4 //!
5 //! This module allows Rust code to use the kernel's [`struct wait_queue_head`] as a condition
6 //! variable.
7
8 use super::{lock::Backend, lock::Guard, LockClassKey};
9 use crate::{
10 init::PinInit,
11 pin_init,
12 str::CStr,
13 task::{MAX_SCHEDULE_TIMEOUT, TASK_INTERRUPTIBLE, TASK_NORMAL, TASK_UNINTERRUPTIBLE},
14 time::Jiffies,
15 types::Opaque,
16 };
17 use core::ffi::{c_int, c_long};
18 use core::marker::PhantomPinned;
19 use core::ptr;
20 use macros::pin_data;
21
22 /// Creates a [`CondVar`] initialiser with the given name and a newly-created lock class.
23 #[macro_export]
24 macro_rules! new_condvar {
25 ($($name:literal)?) => {
26 $crate::sync::CondVar::new($crate::optional_name!($($name)?), $crate::static_lock_class!())
27 };
28 }
29 pub use new_condvar;
30
31 /// A conditional variable.
32 ///
33 /// Exposes the kernel's [`struct wait_queue_head`] as a condition variable. It allows the caller to
34 /// atomically release the given lock and go to sleep. It reacquires the lock when it wakes up. And
35 /// it wakes up when notified by another thread (via [`CondVar::notify_one`] or
36 /// [`CondVar::notify_all`]) or because the thread received a signal. It may also wake up
37 /// spuriously.
38 ///
39 /// Instances of [`CondVar`] need a lock class and to be pinned. The recommended way to create such
40 /// instances is with the [`pin_init`](crate::pin_init) and [`new_condvar`] macros.
41 ///
42 /// # Examples
43 ///
44 /// The following is an example of using a condvar with a mutex:
45 ///
46 /// ```
47 /// use kernel::sync::{new_condvar, new_mutex, CondVar, Mutex};
48 ///
49 /// #[pin_data]
50 /// pub struct Example {
51 /// #[pin]
52 /// value: Mutex<u32>,
53 ///
54 /// #[pin]
55 /// value_changed: CondVar,
56 /// }
57 ///
58 /// /// Waits for `e.value` to become `v`.
59 /// fn wait_for_value(e: &Example, v: u32) {
60 /// let mut guard = e.value.lock();
61 /// while *guard != v {
62 /// e.value_changed.wait(&mut guard);
63 /// }
64 /// }
65 ///
66 /// /// Increments `e.value` and notifies all potential waiters.
67 /// fn increment(e: &Example) {
68 /// *e.value.lock() += 1;
69 /// e.value_changed.notify_all();
70 /// }
71 ///
72 /// /// Allocates a new boxed `Example`.
73 /// fn new_example() -> Result<Pin<Box<Example>>> {
74 /// Box::pin_init(pin_init!(Example {
75 /// value <- new_mutex!(0),
76 /// value_changed <- new_condvar!(),
77 /// }), GFP_KERNEL)
78 /// }
79 /// ```
80 ///
81 /// [`struct wait_queue_head`]: srctree/include/linux/wait.h
82 #[pin_data]
83 pub struct CondVar {
84 #[pin]
85 pub(crate) wait_queue_head: Opaque<bindings::wait_queue_head>,
86
87 /// A condvar needs to be pinned because it contains a [`struct list_head`] that is
88 /// self-referential, so it cannot be safely moved once it is initialised.
89 ///
90 /// [`struct list_head`]: srctree/include/linux/types.h
91 #[pin]
92 _pin: PhantomPinned,
93 }
94
95 // SAFETY: `CondVar` only uses a `struct wait_queue_head`, which is safe to use on any thread.
96 #[allow(clippy::non_send_fields_in_send_ty)]
97 unsafe impl Send for CondVar {}
98
99 // SAFETY: `CondVar` only uses a `struct wait_queue_head`, which is safe to use on multiple threads
100 // concurrently.
101 unsafe impl Sync for CondVar {}
102
103 impl CondVar {
104 /// Constructs a new condvar initialiser.
105 pub fn new(name: &'static CStr, key: &'static LockClassKey) -> impl PinInit<Self> {
106 pin_init!(Self {
107 _pin: PhantomPinned,
108 // SAFETY: `slot` is valid while the closure is called and both `name` and `key` have
109 // static lifetimes so they live indefinitely.
110 wait_queue_head <- Opaque::ffi_init(|slot| unsafe {
111 bindings::__init_waitqueue_head(slot, name.as_char_ptr(), key.as_ptr())
112 }),
113 })
114 }
115
116 fn wait_internal<T: ?Sized, B: Backend>(
117 &self,
118 wait_state: c_int,
119 guard: &mut Guard<'_, T, B>,
120 timeout_in_jiffies: c_long,
121 ) -> c_long {
122 let wait = Opaque::<bindings::wait_queue_entry>::uninit();
123
124 // SAFETY: `wait` points to valid memory.
125 unsafe { bindings::init_wait(wait.get()) };
126
127 // SAFETY: Both `wait` and `wait_queue_head` point to valid memory.
128 unsafe {
129 bindings::prepare_to_wait_exclusive(self.wait_queue_head.get(), wait.get(), wait_state)
130 };
131
132 // SAFETY: Switches to another thread. The timeout can be any number.
133 let ret = guard.do_unlocked(|| unsafe { bindings::schedule_timeout(timeout_in_jiffies) });
134
135 // SAFETY: Both `wait` and `wait_queue_head` point to valid memory.
136 unsafe { bindings::finish_wait(self.wait_queue_head.get(), wait.get()) };
137
138 ret
139 }
140
141 /// Releases the lock and waits for a notification in uninterruptible mode.
142 ///
143 /// Atomically releases the given lock (whose ownership is proven by the guard) and puts the
144 /// thread to sleep, reacquiring the lock on wake up. It wakes up when notified by
145 /// [`CondVar::notify_one`] or [`CondVar::notify_all`]. Note that it may also wake up
146 /// spuriously.
147 pub fn wait<T: ?Sized, B: Backend>(&self, guard: &mut Guard<'_, T, B>) {
148 self.wait_internal(TASK_UNINTERRUPTIBLE, guard, MAX_SCHEDULE_TIMEOUT);
149 }
150
151 /// Releases the lock and waits for a notification in interruptible mode.
152 ///
153 /// Similar to [`CondVar::wait`], except that the wait is interruptible. That is, the thread may
154 /// wake up due to signals. It may also wake up spuriously.
155 ///
156 /// Returns whether there is a signal pending.
157 #[must_use = "wait_interruptible returns if a signal is pending, so the caller must check the return value"]
158 pub fn wait_interruptible<T: ?Sized, B: Backend>(&self, guard: &mut Guard<'_, T, B>) -> bool {
159 self.wait_internal(TASK_INTERRUPTIBLE, guard, MAX_SCHEDULE_TIMEOUT);
160 crate::current!().signal_pending()
161 }
162
163 /// Releases the lock and waits for a notification in interruptible mode.
164 ///
165 /// Atomically releases the given lock (whose ownership is proven by the guard) and puts the
166 /// thread to sleep. It wakes up when notified by [`CondVar::notify_one`] or
167 /// [`CondVar::notify_all`], or when a timeout occurs, or when the thread receives a signal.
168 #[must_use = "wait_interruptible_timeout returns if a signal is pending, so the caller must check the return value"]
169 pub fn wait_interruptible_timeout<T: ?Sized, B: Backend>(
170 &self,
171 guard: &mut Guard<'_, T, B>,
172 jiffies: Jiffies,
173 ) -> CondVarTimeoutResult {
174 let jiffies = jiffies.try_into().unwrap_or(MAX_SCHEDULE_TIMEOUT);
175 let res = self.wait_internal(TASK_INTERRUPTIBLE, guard, jiffies);
176
177 match (res as Jiffies, crate::current!().signal_pending()) {
178 (jiffies, true) => CondVarTimeoutResult::Signal { jiffies },
179 (0, false) => CondVarTimeoutResult::Timeout,
180 (jiffies, false) => CondVarTimeoutResult::Woken { jiffies },
181 }
182 }
183
184 /// Calls the kernel function to notify the appropriate number of threads.
185 fn notify(&self, count: c_int) {
186 // SAFETY: `wait_queue_head` points to valid memory.
187 unsafe {
188 bindings::__wake_up(
189 self.wait_queue_head.get(),
190 TASK_NORMAL,
191 count,
192 ptr::null_mut(),
193 )
194 };
195 }
196
197 /// Calls the kernel function to notify one thread synchronously.
198 ///
199 /// This method behaves like `notify_one`, except that it hints to the scheduler that the
200 /// current thread is about to go to sleep, so it should schedule the target thread on the same
201 /// CPU.
202 pub fn notify_sync(&self) {
203 // SAFETY: `wait_queue_head` points to valid memory.
204 unsafe { bindings::__wake_up_sync(self.wait_queue_head.get(), TASK_NORMAL) };
205 }
206
207 /// Wakes a single waiter up, if any.
208 ///
209 /// This is not 'sticky' in the sense that if no thread is waiting, the notification is lost
210 /// completely (as opposed to automatically waking up the next waiter).
211 pub fn notify_one(&self) {
212 self.notify(1);
213 }
214
215 /// Wakes all waiters up, if any.
216 ///
217 /// This is not 'sticky' in the sense that if no thread is waiting, the notification is lost
218 /// completely (as opposed to automatically waking up the next waiter).
219 pub fn notify_all(&self) {
220 self.notify(0);
221 }
222 }
223
224 /// The return type of `wait_timeout`.
225 pub enum CondVarTimeoutResult {
226 /// The timeout was reached.
227 Timeout,
228 /// Somebody woke us up.
229 Woken {
230 /// Remaining sleep duration.
231 jiffies: Jiffies,
232 },
233 /// A signal occurred.
234 Signal {
235 /// Remaining sleep duration.
236 jiffies: Jiffies,
237 },
238 }
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