Re: [PATCH 3/5] rust: sync: add WaitQueue infrastructure

["Onur Özkan" <work@onurozkan.dev>]

On Mon, 27 Jul 2026 00:36:09 +0200
Danilo Krummrich <dakr@kernel.org> wrote:

> Implement a wait queue wrapping the kernel's struct wait_queue_head,
> with wait_event()-style methods that take a condition closure directly.
> 
> The API mirrors the C wait_event() family:
> 
>   - wait_event()
>   - wait_event_interruptible()
>   - wait_event_timeout()
>   - wait_event_interruptible_timeout()
>   - wake_up() / wake_up_all() / wake_up_sync()
> 
> Interruptible and timeout variants return Result<(), WaitError>, where
> WaitError maps to ERESTARTSYS (signal) or ETIMEDOUT (timeout).
> 
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> ---
>  rust/kernel/sync.rs      |   6 +
>  rust/kernel/sync/wait.rs | 388 +++++++++++++++++++++++++++++++++++++++
>  2 files changed, 394 insertions(+)
>  create mode 100644 rust/kernel/sync/wait.rs
> 
> diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
> index df4f2604ff9b..31f0999e5747 100644
> --- a/rust/kernel/sync.rs
> +++ b/rust/kernel/sync.rs
> @@ -21,6 +21,7 @@
>  pub mod rcu;
>  mod refcount;
>  mod set_once;
> +mod wait;
>  
>  pub use arc::{Arc, ArcBorrow, UniqueArc};
>  pub use completion::Completion;
> @@ -38,6 +39,11 @@
>  pub use locked_by::LockedBy;
>  pub use refcount::Refcount;
>  pub use set_once::SetOnce;
> +pub use wait::{
> +    new_waitqueue,
> +    WaitError,
> +    WaitQueue, //
> +};
>  
>  /// Represents a lockdep class.
>  ///
> diff --git a/rust/kernel/sync/wait.rs b/rust/kernel/sync/wait.rs
> new file mode 100644
> index 000000000000..ba4ee0f8d4d4
> --- /dev/null
> +++ b/rust/kernel/sync/wait.rs
> @@ -0,0 +1,388 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Wait queue.
> +//!
> +//! C header: [`include/linux/wait.h`](srctree/include/linux/wait.h)
> +
> +use super::LockClassKey;
> +use crate::{
> +    prelude::*,
> +    str::CStr,
> +    task::{
> +        self,
> +        TASK_INTERRUPTIBLE,
> +        TASK_NORMAL,
> +        TASK_UNINTERRUPTIBLE, //
> +    },
> +    time::Jiffies,
> +    types::Opaque,
> +};
> +
> +use core::{
> +    pin::Pin,
> +    ptr, //
> +};
> +
> +/// Creates a [`WaitQueue`] initialiser with the given name and a newly-created lock class.
> +#[macro_export]
> +macro_rules! new_waitqueue {
> +    ($($name:literal)?) => {
> +        $crate::sync::WaitQueue::new(
> +            $crate::optional_name!($($name)?),
> +            $crate::static_lock_class!(),
> +        )
> +    };
> +}
> +pub use new_waitqueue;
> +
> +/// Exposes the kernel's [`struct wait_queue_head`] as a Rust wait queue.
> +///
> +/// A `WaitQueue` allows a thread to sleep until a caller-supplied condition becomes true,
> +/// re-checking the condition on each wake-up. This matches the C `wait_event()` family of macros.
> +///
> +/// For waiting with a lock guard (the condition variable pattern), use [`CondVar`](super::CondVar)
> +/// instead.
> +///
> +/// Instances of `WaitQueue` need a lock class and to be pinned. The recommended way to create such
> +/// instances is with the [`pin_init!`] and [`new_waitqueue!`] macros.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use kernel::sync::{
> +///     atomic::{
> +///         Atomic,
> +///         Relaxed,
> +///     },
> +///     new_waitqueue,
> +///     WaitQueue,
> +/// };
> +///
> +/// #[pin_data]
> +/// pub struct Example {
> +///     value: Atomic<i32>,
> +///     #[pin]
> +///     queue: WaitQueue,
> +/// }
> +///
> +/// fn wait_for_value(e: &Example, v: i32) {
> +///     e.queue.wait_event(|| e.value.load(Relaxed) == v);
> +/// }
> +///
> +/// fn set_value(e: &Example, v: i32) {
> +///     e.value.store(v, Relaxed);
> +///     e.queue.wake_up();
> +/// }
> +/// ```
> +///
> +/// [`struct wait_queue_head`]: srctree/include/linux/wait.h
> +#[pin_data]
> +pub struct WaitQueue {
> +    #[pin]
> +    wait_queue_head: Opaque<bindings::wait_queue_head>,
> +}
> +
> +// SAFETY: `WaitQueue` only uses a `struct wait_queue_head`, which is safe to use on any thread.
> +unsafe impl Send for WaitQueue {}
> +
> +// SAFETY: `WaitQueue` only uses a `struct wait_queue_head`, which is safe to use on multiple
> +// threads concurrently.
> +unsafe impl Sync for WaitQueue {}
> +
> +impl WaitQueue {
> +    /// Constructs a new wait queue initialiser.
> +    pub fn new(name: &'static CStr, key: Pin<&'static LockClassKey>) -> impl PinInit<Self> {
> +        pin_init!(Self {
> +            // SAFETY: `slot` is valid while the closure is called and both `name` and `key` have
> +            // static lifetimes so they live indefinitely.
> +            wait_queue_head <- Opaque::ffi_init(|slot| unsafe {
> +                bindings::__init_waitqueue_head(slot, name.as_char_ptr(), key.as_ptr())
> +            }),
> +        })
> +    }
> +
> +    /// Returns a raw pointer to the underlying `wait_queue_head`.
> +    #[expect(unused)]
> +    #[inline]
> +    pub(super) fn as_raw(&self) -> *mut bindings::wait_queue_head {
> +        self.wait_queue_head.get()
> +    }
> +
> +    /// Sleeps until the condition returns `true`.
> +    ///
> +    /// The condition is checked before each sleep and after each wake-up. The wait is
> +    /// uninterruptible.
> +    #[inline]
> +    pub fn wait_event<F: Fn() -> bool>(&self, condition: F) {
> +        self.wait_event_timeout_internal(TASK_UNINTERRUPTIBLE, &condition, Jiffies::MAX);
> +    }
> +
> +    /// Sleeps until the condition returns `true` or a signal is received.
> +    ///
> +    /// Returns `Ok(())` when the condition is met, or `Err(WaitError::Signal)` if interrupted
> +    /// by a signal.
> +    #[inline]
> +    pub fn wait_event_interruptible<F: Fn() -> bool>(&self, condition: F) -> Result<(), WaitError> {
> +        self.wait_event_timeout_internal(TASK_INTERRUPTIBLE, &condition, Jiffies::MAX);
> +        if !condition() && current!().signal_pending() {
> +            Err(WaitError::Signal)
> +        } else {
> +            Ok(())
> +        }
> +    }
> +
> +    /// Sleeps until the condition returns `true` or the timeout expires.
> +    ///
> +    /// Returns `Ok(())` when the condition is met, or `Err(WaitError::Timeout)` if the timeout
> +    /// elapsed first.
> +    #[inline]
> +    pub fn wait_event_timeout<F: Fn() -> bool>(
> +        &self,
> +        condition: F,
> +        jiffies: Jiffies,
> +    ) -> Result<(), WaitError> {
> +        let remaining = self.wait_event_timeout_internal(TASK_UNINTERRUPTIBLE, &condition, jiffies);
> +        if remaining == 0 && !condition() {
> +            Err(WaitError::Timeout)
> +        } else {
> +            Ok(())
> +        }
> +    }
> +
> +    /// Sleeps until the condition returns `true`, a signal is received, or the timeout expires.
> +    ///
> +    /// Returns `Ok(())` when the condition is met, or `Err(WaitError)` on signal or timeout.
> +    #[inline]
> +    pub fn wait_event_interruptible_timeout<F: Fn() -> bool>(
> +        &self,
> +        condition: F,
> +        jiffies: Jiffies,
> +    ) -> Result<(), WaitError> {
> +        let remaining = self.wait_event_timeout_internal(TASK_INTERRUPTIBLE, &condition, jiffies);
> +        if condition() {
> +            Ok(())
> +        } else if current!().signal_pending() {
> +            Err(WaitError::Signal)
> +        } else if remaining == 0 {
> +            Err(WaitError::Timeout)
> +        } else {
> +            Ok(())
> +        }
> +    }
> +
> +    fn wait_event_timeout_internal(
> +        &self,
> +        wait_state: c_int,
> +        condition: &dyn Fn() -> bool,
> +        jiffies: Jiffies,
> +    ) -> Jiffies {

Would it make sense to check condition() first and return early?

> +        let wait = Opaque::<bindings::wait_queue_entry>::uninit();
> +
> +        // SAFETY: `wait` points to valid memory.
> +        unsafe { bindings::init_wait(wait.get()) };
> +
> +        let mut remaining = jiffies;
> +
> +        loop {
> +            // SAFETY: Both `wait` and `wait_queue_head` point to valid memory, and `wait` was
> +            // initialised by `init_wait()` above.
> +            let ret = unsafe {
> +                bindings::prepare_to_wait_event(self.wait_queue_head.get(), wait.get(), wait_state)
> +            };
> +
> +            if condition() {
> +                break;
> +            }
> +
> +            if ret != 0 || remaining == 0 {
> +                break;
> +            }
> +
> +            remaining = task::schedule_timeout(remaining);
> +
> +            if condition() {
> +                break;
> +            }
> +        }
> +
> +        // SAFETY: Both `wait` and `wait_queue_head` point to valid memory.
> +        unsafe { bindings::finish_wait(self.wait_queue_head.get(), wait.get()) };
> +
> +        remaining
> +    }
> +
> +    /// Performs a single exclusive prepare-to-wait / finish-wait cycle, calling `schedule_fn`
> +    /// in between.
> +    #[expect(unused)]
> +    pub(super) fn wait_once_exclusive<F, R>(&self, wait_state: c_int, schedule_fn: F) -> R
> +    where
> +        F: FnOnce() -> R,
> +    {
> +        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_queue_head` point to valid memory.
> +        unsafe {
> +            bindings::prepare_to_wait_exclusive(self.wait_queue_head.get(), wait.get(), wait_state)
> +        };
> +
> +        let ret = schedule_fn();
> +
> +        // SAFETY: Both `wait` and `wait_queue_head` point to valid memory.
> +        unsafe { bindings::finish_wait(self.wait_queue_head.get(), wait.get()) };
> +
> +        ret
> +    }
> +
> +    /// Wakes up waiters.
> +    ///
> +    /// Wakes all non-exclusive waiters and one exclusive waiter, if any.  Matches C's `wake_up()`.
> +    #[inline]
> +    pub fn wake_up(&self) {
> +        // SAFETY: `wait_queue_head` points to valid memory.
> +        unsafe { bindings::__wake_up(self.wait_queue_head.get(), TASK_NORMAL, 1, ptr::null_mut()) };
> +    }
> +
> +    /// Wakes up all waiters.
> +    ///
> +    /// Wakes all non-exclusive and all exclusive waiters, if any.
> +    /// Matches C's `wake_up_all()`.
> +    #[inline]
> +    pub fn wake_up_all(&self) {
> +        // SAFETY: `wait_queue_head` points to valid memory.
> +        unsafe { bindings::__wake_up(self.wait_queue_head.get(), TASK_NORMAL, 0, ptr::null_mut()) };
> +    }
> +
> +    /// Like [`wake_up()`](Self::wake_up), but hints to the scheduler that the current task is
> +    /// about to sleep, so the woken task should be scheduled on the same CPU to avoid unnecessary
> +    /// migration. Matches C's `wake_up_sync()`.
> +    #[inline]
> +    pub fn wake_up_sync(&self) {
> +        // SAFETY: `wait_queue_head` points to valid memory.
> +        unsafe { bindings::__wake_up_sync(self.wait_queue_head.get(), TASK_NORMAL) };
> +    }
> +
> +    /// Wakes up all waiters and clears poll registrations.
> +    ///
> +    /// Used when a wait queue is about to be freed, to ensure epoll items are properly removed.
> +    /// Matches C's `wake_up_pollfree()`.
> +    #[inline]
> +    #[expect(unused)]
> +    pub(super) fn wake_up_pollfree(&self) {
> +        // SAFETY: `wait_queue_head` points to valid memory.
> +        unsafe { bindings::__wake_up_pollfree(self.wait_queue_head.get()) };
> +    }
> +}
> +
> +/// Error returned by [`WaitQueue`] wait functions.
> +#[derive(Debug, PartialEq)]
> +pub enum WaitError {
> +    /// Interrupted by a signal.
> +    Signal,
> +    /// The timeout elapsed without the condition being met.
> +    Timeout,
> +}
> +
> +impl From<WaitError> for Error {
> +    #[inline]
> +    fn from(e: WaitError) -> Error {
> +        match e {
> +            WaitError::Signal => ERESTARTSYS,
> +            WaitError::Timeout => ETIMEDOUT,
> +        }
> +    }
> +}
> +
> +#[macros::kunit_tests(rust_waitqueue)]
> +mod tests {
> +    use super::*;
> +    use crate::{
> +        sync::{
> +            atomic::{
> +                Atomic,
> +                Relaxed, //
> +            },
> +            Arc,
> +        },
> +        time::{
> +            delay::fsleep,
> +            Delta, //
> +        },
> +        workqueue,
> +    };
> +
> +    #[pin_data]
> +    struct State {
> +        value: Atomic<i32>,
> +        #[pin]
> +        wq: WaitQueue,
> +    }
> +
> +    impl State {
> +        fn new() -> Result<Arc<Self>> {
> +            Arc::pin_init(
> +                pin_init!(Self {
> +                    value: Atomic::new(0),
> +                    wq <- new_waitqueue!(),
> +                }),
> +                GFP_KERNEL,
> +            )
> +        }
> +    }
> +
> +    #[test]
> +    fn wait_event_from_work() {
> +        let s = State::new().unwrap();
> +
> +        let s2 = s.clone();
> +        workqueue::system_dfl()
> +            .try_spawn(GFP_KERNEL, move || {
> +                s2.value.store(1, Relaxed);
> +                s2.wq.wake_up();
> +            })
> +            .unwrap();
> +
> +        s.wq.wait_event(|| s.value.load(Relaxed) == 1);
> +        assert_eq!(s.value.load(Relaxed), 1);
> +    }
> +
> +    #[test]
> +    fn wait_event_condition_already_true() {
> +        let s = State::new().unwrap();
> +        s.value.store(1, Relaxed);
> +
> +        s.wq.wait_event(|| s.value.load(Relaxed) == 1);
> +        assert_eq!(s.value.load(Relaxed), 1);
> +    }
> +
> +    #[test]
> +    fn wait_event_condition_not_yet_met() {
> +        let s = State::new().unwrap();
> +
> +        let s2 = s.clone();
> +        workqueue::system_dfl()
> +            .try_spawn(GFP_KERNEL, move || {
> +                s2.value.store(1, Relaxed);
> +                s2.wq.wake_up_all();
> +
> +                fsleep(Delta::from_millis(50));
> +
> +                s2.value.store(2, Relaxed);
> +                s2.wq.wake_up_all();
> +            })
> +            .unwrap();
> +
> +        s.wq.wait_event(|| s.value.load(Relaxed) == 2);
> +        assert_eq!(s.value.load(Relaxed), 2);
> +    }
> +
> +    #[test]
> +    fn wait_event_timeout_expires() {
> +        let s = State::new().unwrap();
> +
> +        let ret = s.wq.wait_event_timeout(|| s.value.load(Relaxed) == 1, 1);
> +        assert_eq!(ret, Err(WaitError::Timeout));
> +    }
> +}
> -- 
> 2.55.0
>