[PATCH v2 2/5] rust: sync: add WaitQueue infrastructure

[Danilo Krummrich <dakr@kernel.org>]

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()

The interruptible variant returns Result<(), Interrupted>, the timeout
variant Result<(), TimedOut>, and the combined variant Result<(),
WaitError>. These errors map to ERESTARTSYS (signal) or ETIMEDOUT
(timeout).

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/sync.rs      |   8 +
 rust/kernel/sync/wait.rs | 527 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 535 insertions(+)
 create mode 100644 rust/kernel/sync/wait.rs

diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
index ee372c048a8a..5f7ffd828259 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -19,6 +19,7 @@
 mod refcount;
 mod set_once;
 pub mod srcu;
+mod wait;
 
 pub use arc::{Arc, ArcBorrow, UniqueArc};
 pub use completion::Completion;
@@ -41,3 +42,10 @@
 pub use refcount::Refcount;
 pub use set_once::SetOnce;
 pub use srcu::Srcu;
+pub use wait::{
+    new_waitqueue,
+    Interrupted,
+    TimedOut,
+    WaitError,
+    WaitQueue, //
+};
diff --git a/rust/kernel/sync/wait.rs b/rust/kernel/sync/wait.rs
new file mode 100644
index 000000000000..00120dbb4fca
--- /dev/null
+++ b/rust/kernel/sync/wait.rs
@@ -0,0 +1,527 @@
+// SPDX-License-Identifier: GPL-2.0
+// SPDX-FileCopyrightText: Copyright (c) 2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+
+//! Wait queue.
+//!
+//! C header: [`include/linux/wait.h`](srctree/include/linux/wait.h)
+
+use super::LockClassKey;
+use crate::{
+    prelude::*,
+    str::CStr,
+    task,
+    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`.
+    #[inline]
+    pub 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: FnMut() -> bool>(&self, condition: F) {
+        // An uninterruptible wait without a timeout cannot fail.
+        let _ =
+            self.wait_event_timeout_internal(task::TASK_UNINTERRUPTIBLE, condition, Jiffies::MAX);
+    }
+
+    /// Sleeps until the condition returns `true` or a signal is received.
+    ///
+    /// Returns `Ok(())` when the condition is met, or [`Interrupted`] if interrupted by a signal.
+    #[inline]
+    pub fn wait_event_interruptible<F: FnMut() -> bool>(
+        &self,
+        condition: F,
+    ) -> Result<(), Interrupted> {
+        // Without a timeout, interruption is the only possible error.
+        self.wait_event_timeout_internal(task::TASK_INTERRUPTIBLE, condition, Jiffies::MAX)
+            .map_err(|_| Interrupted)
+    }
+
+    /// Sleeps until the condition returns `true` or the timeout expires.
+    ///
+    /// Returns `Ok(())` when the condition is met, or [`TimedOut`] if the timeout elapsed first.
+    #[inline]
+    pub fn wait_event_timeout<F: FnMut() -> bool>(
+        &self,
+        condition: F,
+        jiffies: Jiffies,
+    ) -> Result<(), TimedOut> {
+        // An uninterruptible wait can only fail by timing out.
+        self.wait_event_timeout_internal(task::TASK_UNINTERRUPTIBLE, condition, jiffies)
+            .map_err(|_| TimedOut)
+    }
+
+    /// Sleeps until the condition returns `true`, a signal is received, or the timeout expires.
+    ///
+    /// Returns `Ok(())` when the condition is met, or [`WaitError`] on signal or timeout.
+    #[inline]
+    pub fn wait_event_interruptible_timeout<F: FnMut() -> bool>(
+        &self,
+        condition: F,
+        jiffies: Jiffies,
+    ) -> Result<(), WaitError> {
+        self.wait_event_timeout_internal(task::TASK_INTERRUPTIBLE, condition, jiffies)
+    }
+
+    fn wait_event_timeout_internal<F: FnMut() -> bool>(
+        &self,
+        wait_state: c_int,
+        mut condition: F,
+        jiffies: Jiffies,
+    ) -> Result<(), WaitError> {
+        if condition() {
+            return Ok(());
+        }
+
+        if jiffies == 0 {
+            return Err(TimedOut.into());
+        }
+
+        let wait = Opaque::<bindings::wait_queue_entry>::uninit();
+
+        // SAFETY: `wait` points to valid memory.
+        unsafe { bindings::init_wait(wait.get()) };
+
+        let mut remaining = jiffies;
+
+        let result = loop {
+            // SAFETY: Both pointers are valid, 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 Ok(());
+            }
+
+            if ret != 0 {
+                break Err(Interrupted.into());
+            }
+
+            remaining = task::schedule_timeout(remaining);
+
+            if condition() {
+                break Ok(());
+            }
+
+            if remaining == 0 {
+                break Err(TimedOut.into());
+            }
+        };
+
+        // SAFETY: Both pointers are valid, and `wait` is no longer used after this call.
+        unsafe { bindings::finish_wait(self.wait_queue_head.get(), wait.get()) };
+
+        result
+    }
+
+    /// Performs a single non-exclusive prepare-to-wait / finish-wait cycle, calling `schedule_fn`
+    /// in between. Non-exclusive waiters are all woken by [`wake_up()`](Self::wake_up).
+    #[inline]
+    #[expect(unused)]
+    pub(super) fn wait_once<F, R>(&self, wait_state: c_int, schedule_fn: F) -> R
+    where
+        F: FnOnce() -> R,
+    {
+        self.wait_once_internal(wait_state, false, schedule_fn)
+    }
+
+    /// Performs a single exclusive prepare-to-wait / finish-wait cycle, calling `schedule_fn`
+    /// in between.
+    #[inline]
+    #[expect(unused)]
+    pub(super) fn wait_once_exclusive<F, R>(&self, wait_state: c_int, schedule_fn: F) -> R
+    where
+        F: FnOnce() -> R,
+    {
+        self.wait_once_internal(wait_state, true, schedule_fn)
+    }
+
+    fn wait_once_internal<F, R>(&self, wait_state: c_int, exclusive: bool, 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 {
+            if exclusive {
+                bindings::prepare_to_wait_exclusive(
+                    self.wait_queue_head.get(),
+                    wait.get(),
+                    wait_state,
+                );
+            } else {
+                bindings::prepare_to_wait(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(),
+                bindings::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(),
+                bindings::TASK_NORMAL,
+                0,
+                ptr::null_mut(),
+            )
+        };
+    }
+
+    /// Like [`wake_up()`](Self::wake_up), but hints to the scheduler that the current task
+    /// expects to sleep soon. 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(), bindings::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()) };
+    }
+}
+
+/// A wait was interrupted by a signal.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub struct Interrupted;
+
+/// A wait timed out before its condition was met.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub struct TimedOut;
+
+impl From<Interrupted> for Error {
+    #[inline]
+    fn from(_: Interrupted) -> Error {
+        ERESTARTSYS
+    }
+}
+
+impl From<TimedOut> for Error {
+    #[inline]
+    fn from(_: TimedOut) -> Error {
+        ETIMEDOUT
+    }
+}
+
+/// Error returned by [`WaitQueue`] wait functions.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum WaitError {
+    /// Interrupted by a signal.
+    Signal,
+    /// The timeout elapsed without the condition being met.
+    Timeout,
+}
+
+impl From<Interrupted> for WaitError {
+    #[inline]
+    fn from(_: Interrupted) -> Self {
+        Self::Signal
+    }
+}
+
+impl From<TimedOut> for WaitError {
+    #[inline]
+    fn from(_: TimedOut) -> Self {
+        Self::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();
+        let mut calls = 0;
+
+        s.wq.wait_event(|| {
+            calls += 1;
+            true
+        });
+
+        assert_eq!(calls, 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(TimedOut));
+    }
+
+    #[test]
+    fn wait_event_does_not_recheck_success() {
+        enum WaitKind {
+            Uninterruptible,
+            Interruptible,
+            Timeout,
+            InterruptibleTimeout,
+        }
+
+        let s = State::new().unwrap();
+
+        for kind in [
+            WaitKind::Uninterruptible,
+            WaitKind::Interruptible,
+            WaitKind::Timeout,
+            WaitKind::InterruptibleTimeout,
+        ] {
+            let mut calls = 0;
+            let condition = || {
+                // Bypass the initial fast path to exercise the wait loop.
+                calls += 1;
+                calls > 1
+            };
+
+            let ret = match kind {
+                WaitKind::Uninterruptible => {
+                    s.wq.wait_event(condition);
+                    Ok(())
+                }
+                WaitKind::Interruptible => {
+                    s.wq.wait_event_interruptible(condition).map_err(Into::into)
+                }
+                WaitKind::Timeout => s.wq.wait_event_timeout(condition, 1).map_err(Into::into),
+                WaitKind::InterruptibleTimeout => {
+                    s.wq.wait_event_interruptible_timeout(condition, 1)
+                }
+            };
+
+            assert_eq!(ret, Ok(()));
+            assert_eq!(calls, 2);
+        }
+    }
+
+    #[test]
+    fn wait_event_zero_timeout() {
+        let s = State::new().unwrap();
+
+        for interruptible in [false, true] {
+            let mut calls = 0;
+            let condition = || {
+                calls += 1;
+                false
+            };
+
+            let ret = if interruptible {
+                s.wq.wait_event_interruptible_timeout(condition, 0)
+            } else {
+                s.wq.wait_event_timeout(condition, 0).map_err(Into::into)
+            };
+
+            assert_eq!(ret, Err(WaitError::Timeout));
+            assert_eq!(calls, 1);
+        }
+    }
+}
-- 
2.56.0