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// SPDX-License-Identifier: GPL-2.0
//! A condition variable.
//!
//! This module allows Rust code to use the kernel's [`struct wait_queue_head`] as a condition
//! variable.
use super::{Guard, Lock, LockClassKey, LockInfo, NeedsLockClass};
use crate::{bindings, str::CStr, task::Task, Opaque};
use core::{marker::PhantomPinned, pin::Pin};
/// Safely initialises a [`CondVar`] with the given name, generating a new lock class.
#[macro_export]
macro_rules! condvar_init {
($condvar:expr, $name:literal) => {
$crate::init_with_lockdep!($condvar, $name)
};
}
// TODO: `bindgen` is not generating this constant. Figure out why.
const POLLFREE: u32 = 0x4000;
/// Exposes the kernel's [`struct wait_queue_head`] as a condition variable. It allows the caller to
/// atomically release the given lock and go to sleep. It reacquires the lock when it wakes up. And
/// it wakes up when notified by another thread (via [`CondVar::notify_one`] or
/// [`CondVar::notify_all`]) or because the thread received a signal.
///
/// [`struct wait_queue_head`]: ../../../include/linux/wait.h
pub struct CondVar {
pub(crate) wait_list: Opaque<bindings::wait_queue_head>,
/// A condvar needs to be pinned because it contains a [`struct list_head`] that is
/// self-referential, so it cannot be safely moved once it is initialised.
_pin: PhantomPinned,
}
// SAFETY: `CondVar` only uses a `struct wait_queue_head`, which is safe to use on any thread.
#[allow(clippy::non_send_fields_in_send_ty)]
unsafe impl Send for CondVar {}
// SAFETY: `CondVar` only uses a `struct wait_queue_head`, which is safe to use on multiple threads
// concurrently.
unsafe impl Sync for CondVar {}
impl CondVar {
/// Constructs a new conditional variable.
///
/// # Safety
///
/// The caller must call `CondVar::init` before using the conditional variable.
pub const unsafe fn new() -> Self {
Self {
wait_list: Opaque::uninit(),
_pin: PhantomPinned,
}
}
/// Atomically releases the given lock (whose ownership is proven by the guard) and puts the
/// thread to sleep. It wakes up when notified by [`CondVar::notify_one`] or
/// [`CondVar::notify_all`], or when the thread receives a signal.
///
/// Returns whether there is a signal pending.
#[must_use = "wait returns if a signal is pending, so the caller must check the return value"]
pub fn wait<L: Lock<I>, I: LockInfo>(&self, guard: &mut Guard<'_, L, I>) -> bool {
let lock = guard.lock;
let wait = Opaque::<bindings::wait_queue_entry>::uninit();
// SAFETY: `wait` points to valid memory.
unsafe { bindings::init_wait(wait.get()) };
// SAFETY: Both `wait` and `wait_list` point to valid memory.
unsafe {
bindings::prepare_to_wait_exclusive(
self.wait_list.get(),
wait.get(),
bindings::TASK_INTERRUPTIBLE as _,
)
};
// SAFETY: The guard is evidence that the caller owns the lock.
unsafe { lock.unlock(&mut guard.context) };
// SAFETY: No arguments, switches to another thread.
unsafe { bindings::schedule() };
guard.context = lock.lock_noguard();
// SAFETY: Both `wait` and `wait_list` point to valid memory.
unsafe { bindings::finish_wait(self.wait_list.get(), wait.get()) };
Task::current().signal_pending()
}
/// Calls the kernel function to notify the appropriate number of threads with the given flags.
fn notify(&self, count: i32, flags: u32) {
// SAFETY: `wait_list` points to valid memory.
unsafe {
bindings::__wake_up(
self.wait_list.get(),
bindings::TASK_NORMAL,
count,
flags as _,
)
};
}
/// Wakes a single waiter up, if any. This is not 'sticky' in the sense that if no thread is
/// waiting, the notification is lost completely (as opposed to automatically waking up the
/// next waiter).
pub fn notify_one(&self) {
self.notify(1, 0);
}
/// Wakes all waiters up, if any. This is not 'sticky' in the sense that if no thread is
/// waiting, the notification is lost completely (as opposed to automatically waking up the
/// next waiter).
pub fn notify_all(&self) {
self.notify(0, 0);
}
/// Wakes all waiters up. If they were added by `epoll`, they are also removed from the list of
/// waiters. This is useful when cleaning up a condition variable that may be waited on by
/// threads that use `epoll`.
pub fn free_waiters(&self) {
self.notify(1, bindings::POLLHUP | POLLFREE);
}
}
impl NeedsLockClass for CondVar {
fn init(
self: Pin<&mut Self>,
name: &'static CStr,
key: &'static LockClassKey,
_: &'static LockClassKey,
) {
unsafe {
bindings::__init_waitqueue_head(self.wait_list.get(), name.as_char_ptr(), key.get())
};
}
}
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