Re: [PATCH v12 4/6] rust: ww_mutex: add Mutex, AcquireCtx and MutexGuard

["Gary Guo" <gary@garyguo.net>]

On Mon Oct 12, 2026 at 11:26 AM BST, Onur Özkan wrote:
> Covers the low-level locking API (lock, try_lock and
> interruptible variants) and integration with kernel bindings.
>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Onur Özkan <work@onurozkan.dev>
> ---
>  rust/kernel/sync/lock/ww_mutex.rs             | 434 ++++++++++++++++++
>  rust/kernel/sync/lock/ww_mutex/acquire_ctx.rs | 149 ++++++
>  2 files changed, 583 insertions(+)
>  create mode 100644 rust/kernel/sync/lock/ww_mutex/acquire_ctx.rs
>
> diff --git a/rust/kernel/sync/lock/ww_mutex.rs b/rust/kernel/sync/lock/ww_mutex.rs
> index 727c51cc73af8..2423f89a7e762 100644
> --- a/rust/kernel/sync/lock/ww_mutex.rs
> +++ b/rust/kernel/sync/lock/ww_mutex.rs
> @@ -1,7 +1,441 @@
>  // SPDX-License-Identifier: GPL-2.0
>  
>  //! Rust abstractions for the kernel's wound-wait locking primitives.
> +//!
> +//! It is designed to avoid deadlocks when locking multiple [`Mutex`]es
> +//! that belong to the same [`Class`]. Each lock acquisition uses an
> +//! [`AcquireCtx`] to track ordering and ensure forward progress.
> +//!
> +//! See srctree/Documentation/locking/ww-mutex-design.rst for more details.
>  
> +use crate::{
> +    bindings,
> +    container_of,
> +    error::to_result,
> +    prelude::*,
> +    types::{
> +        NotThreadSafe,
> +        Opaque, //
> +    }, //
> +};
> +use core::{
> +    cell::UnsafeCell,
> +    marker::PhantomData, //
> +};
> +
> +pub use acquire_ctx::AcquireCtx;
>  pub use class::Class;
>  
> +mod acquire_ctx;
>  mod class;
> +
> +/// A wound-wait (ww) mutex that is powered with deadlock avoidance
> +/// when acquiring multiple locks of the same [`Class`].
> +///
> +/// Each mutex belongs to a [`Class`], which the wound-wait algorithm
> +/// uses to figure out the order of acquisition and prevent deadlocks.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use kernel::{
> +///     define_ww_class,
> +///     sync::{
> +///         lock::ww_mutex::{
> +///             AcquireCtx,
> +///             Class,
> +///             Mutex, //
> +///         },
> +///         Arc,
> +///     },
> +/// };
> +/// use pin_init::stack_pin_init;
> +///
> +/// define_ww_class!(SOME_WW_CLASS);
> +///
> +/// let mutex = Arc::pin_init(Mutex::new(42, &SOME_WW_CLASS), GFP_KERNEL)?;
> +/// let ctx = KBox::pin_init(AcquireCtx::new(&SOME_WW_CLASS), GFP_KERNEL)?;
> +///
> +/// // SAFETY: The guard is dropped before `ctx`.
> +/// let guard = unsafe { ctx.lock(&mutex) }?;
> +/// assert_eq!(*guard, 42);
> +///
> +/// # Ok::<(), Error>(())
> +/// ```
> +#[pin_data]
> +#[repr(C)]
> +pub struct Mutex<'a, T: ?Sized> {
> +    _p: PhantomData<&'a Class>,

Do you expect any users that have non-static class?

So far all in-kernel user of ww mutex has a static class.

> +    #[pin]
> +    inner: Opaque<bindings::ww_mutex>,
> +    data: UnsafeCell<T>,
> +}
> +
> +impl<'class, T> Mutex<'class, T> {
> +    /// Initializes [`Mutex`] with the given `data` and [`Class`].
> +    pub fn new(data: T, class: &'class Class) -> impl PinInit<Self> {
> +        let class_ptr = class.inner.get();
> +        pin_init!(Mutex {
> +            inner <- Opaque::ffi_init(|slot: *mut bindings::ww_mutex| {
> +                // SAFETY: `class` is valid for the lifetime `'class` captured by `Self`.
> +                unsafe { bindings::ww_mutex_init(slot, class_ptr) }
> +            }),
> +            data: UnsafeCell::new(data),
> +            _p: PhantomData
> +        })
> +    }
> +}
> +
> +impl<'class, T: ?Sized> Mutex<'class, T> {
> +    /// Checks if this [`Mutex`] is currently locked.
> +    ///
> +    /// The returned value is racy as another thread can acquire
> +    /// or release the lock immediately after this call returns.
> +    #[inline]
> +    pub fn is_locked(&self) -> bool {
> +        // SAFETY: It's safe to call `ww_mutex_is_locked` on
> +        // a valid mutex.
> +        unsafe { bindings::ww_mutex_is_locked(self.inner.get()) }
> +    }
> +
> +    /// Locks this [`Mutex`] without [`AcquireCtx`].
> +    pub fn lock(&self) -> Result<MutexGuard<'_, T>> {
> +        lock_common(self, None, LockKind::Regular)
> +    }
> +
> +    /// Similar to [`Self::lock`], but can be interrupted by signals.
> +    pub fn lock_interruptible(&self) -> Result<MutexGuard<'_, T>> {
> +        lock_common(self, None, LockKind::Interruptible)
> +    }
> +
> +    /// Tries to lock this [`Mutex`] with no [`AcquireCtx`] and without blocking.
> +    ///
> +    /// Unlike [`Self::lock`], no deadlock handling is performed.
> +    pub fn try_lock(&self) -> Result<MutexGuard<'_, T>> {

`try_lock` failure should return `Option` instead.

> +        lock_common(self, None, LockKind::Try)
> +    }
> +}
> +
> +impl<'class> Mutex<'class, ()> {
> +    /// Creates a [`Mutex`] from a raw pointer.
> +    ///
> +    /// This function is intended for interoperability with C code.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller must ensure that:
> +    ///
> +    /// - `ptr` is a valid pointer to a `ww_mutex`.
> +    /// - `ptr` must remain valid for the lifetime `'a`.
> +    /// - ww_class associated with this mutex must be valid for
> +    ///   the lifetime `'class`.
> +    pub unsafe fn from_raw<'a>(ptr: *mut bindings::ww_mutex) -> &'a Self {
> +        // SAFETY: By the safety contract, the caller guarantees that `ptr`
> +        // points to a valid `ww_mutex` which is the `inner` field of `Mutex`,
> +        // that it remains valid for the lifetime `'a` and the associated
> +        // ww_class outlives `'class`.
> +        //
> +        // Because [`Mutex`] is `#[repr(C)]`, the `inner` field sits at a
> +        // stable offset that `container_of!` can safely rely on.
> +        unsafe { &*container_of!(Opaque::cast_from(ptr), Self, inner) }
> +    }
> +}
> +
> +// SAFETY: `Mutex` can be sent to another thread if the protected
> +// data `T` can be.
> +unsafe impl<T: ?Sized + Send> Send for Mutex<'_, T> {}
> +
> +// SAFETY: `Mutex` serialises access to the protected data, so it is
> +// `Sync` as long as the data is `Send`.
> +unsafe impl<T: ?Sized + Send> Sync for Mutex<'_, T> {}
> +
> +/// A guard that provides exclusive access to the data protected
> +/// by a [`Mutex`].
> +///
> +/// # Invariants
> +///
> +/// The guard holds an exclusive lock on the associated [`Mutex`]. The lock is held
> +/// for the entire lifetime of this guard and is automatically released when the
> +/// guard is dropped.
> +#[must_use = "the lock unlocks immediately when the guard is unused"]
> +pub struct MutexGuard<'a, T: ?Sized> {
> +    mutex: &'a Mutex<'a, T>,
> +    _not_send: NotThreadSafe,
> +}
> +
> +impl<'a, T: ?Sized> MutexGuard<'a, T> {
> +    /// Creates a new guard for the given [`Mutex`].
> +    fn new(mutex: &'a Mutex<'a, T>) -> Self {
> +        assert!(mutex.is_locked());
> +
> +        // INVARIANT: `lock_common` and `from_raw` ensure that the current task holds the mutex
> +        // and transfer sole responsibility for unlocking it to this guard.
> +        Self {
> +            mutex,
> +            _not_send: NotThreadSafe,
> +        }
> +    }
> +}
> +
> +impl<'a> MutexGuard<'a, ()> {
> +    /// Creates a [`MutexGuard`] from a raw pointer.
> +    ///
> +    /// If the given pointer refers to a mutex that is not locked,
> +    /// returns [`EINVAL`].
> +    ///
> +    /// This function is intended for interoperability with C code.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller must ensure that:
> +    ///
> +    /// - `ptr` is a valid pointer to a `ww_mutex`.
> +    /// - `ptr` must remain valid for the lifetime `'b`.
> +    /// - The `ww_class` associated with the `ww_mutex` must be valid for the lifetime `'b`.
> +    /// - The current task holds the mutex.
> +    /// - The caller transfers responsibility for unlocking the mutex to the returned guard
> +    ///   and must not unlock it separately (including through another guard).
> +    /// - Any associated acquire context remains valid until the mutex is unlocked,
> +    ///   even if the returned guard is forgotten. The context must not be finalized or
> +    ///   reinitialized before then.
> +    pub unsafe fn from_raw<'b>(ptr: *mut bindings::ww_mutex) -> Result<MutexGuard<'b, ()>> {
> +        // SAFETY: By this function's safety contract, the caller guarantees that `ptr` points to a
> +        // valid `ww_mutex` which is the `inner` field of a `Mutex`. The caller also guarantees
> +        // that both `ptr` and the associated `ww_class` are valid for the lifetime `'b`.
> +        let mutex = unsafe { Mutex::from_raw(ptr) };
> +
> +        if !mutex.is_locked() {
> +            return Err(EINVAL);
> +        }

Why is this checked despite being an unsafe function. Do you expect users to
call this without knowing the lock is already locked? I think that's unlikely.
Perhaps make this a debug_assert instead?

> +
> +        Ok(MutexGuard::new(mutex))
> +    }
> +}
> +
> +impl<T: ?Sized> core::ops::Deref for MutexGuard<'_, T> {
> +    type Target = T;
> +

`#[inline]` for this and others.

> +    fn deref(&self) -> &Self::Target {
> +        // SAFETY: self.mutex is locked, so we have exclusive access.
> +        unsafe { &*self.mutex.data.get() }
> +    }
> +}
> +
> +impl<T: ?Sized + Unpin> core::ops::DerefMut for MutexGuard<'_, T> {
> +    fn deref_mut(&mut self) -> &mut Self::Target {
> +        // SAFETY: self.mutex is locked, so we have exclusive access.
> +        unsafe { &mut *self.mutex.data.get() }
> +    }
> +}
> +
> +impl<T: ?Sized> Drop for MutexGuard<'_, T> {
> +    #[inline]
> +    fn drop(&mut self) {
> +        // SAFETY: self.mutex is locked and are about to release it.
> +        unsafe { bindings::ww_mutex_unlock(self.mutex.inner.get()) };
> +    }
> +}
> +
> +// SAFETY: `MutexGuard` can be shared between threads if the data can.
> +unsafe impl<T: ?Sized + Sync> Sync for MutexGuard<'_, T> {}
> +
> +/// Locking kinds used by [`lock_common`] to unify the internal
> +/// locking logic.
> +///
> +/// It's best not to expose this type (and [`lock_common`]) to the
> +/// kernel, as it allows internal API changes without worrying
> +/// about breaking external compatibility.
> +#[derive(Copy, Clone, Debug)]
> +enum LockKind {
> +    /// Blocks until lock is acquired.
> +    Regular,
> +    /// Blocks but can be interrupted by signals.
> +    Interruptible,
> +    /// Does not block, returns immediately if busy.
> +    Try,
> +}
> +
> +/// Internal helper that unifies the different locking kinds.
> +///
> +/// Returns [`EINVAL`] if the [`Mutex`] has a different [`Class`].
> +fn lock_common<'a, T: ?Sized>(
> +    mutex: &'a Mutex<'a, T>,
> +    ctx: Option<&AcquireCtx<'_>>,
> +    kind: LockKind,
> +) -> Result<MutexGuard<'a, T>> {

I think we should have 3 helpers, to avoid having to branch on `kind` which is
compile-tiome known information.

The only part between the 3 kinds that is shared is the ctx_ptr computation. I
think it should be extracted to its own helper, and be used for 3 types of
locking.

> +    let mutex_ptr = mutex.inner.get();
> +
> +    let ctx_ptr = match ctx {
> +        Some(acquire_ctx) => {
> +            let ctx_ptr = acquire_ctx.inner.get();
> +
> +            // SAFETY: `ctx_ptr` is a valid pointer for the entire
> +            // lifetime of `ctx`.
> +            let ctx_class = unsafe { (*ctx_ptr).ww_class };
> +
> +            // SAFETY: `mutex_ptr` is a valid pointer for the entire
> +            // lifetime of `mutex`.
> +            let mutex_class = unsafe { (*mutex_ptr).ww_class };
> +
> +            // `ctx` and `mutex` must use the same class.
> +            if ctx_class != mutex_class {
> +                return Err(EINVAL);
> +            }

This is supposed to be a error condition?

> +
> +            ctx_ptr
> +        }
> +        None => core::ptr::null_mut(),
> +    };
> +
> +    match kind {
> +        LockKind::Regular => {
> +            // SAFETY: `Mutex` is always pinned. If `AcquireCtx` is `Some`, it is pinned,
> +            // if `None`, it is set to `core::ptr::null_mut()`. Both cases are safe.
> +            let ret = unsafe { bindings::ww_mutex_lock(mutex_ptr, ctx_ptr) };
> +
> +            to_result(ret)?;
> +        }
> +        LockKind::Interruptible => {
> +            // SAFETY: `Mutex` is always pinned. If `AcquireCtx` is `Some`, it is pinned,
> +            // if `None`, it is set to `core::ptr::null_mut()`. Both cases are safe.
> +            let ret = unsafe { bindings::ww_mutex_lock_interruptible(mutex_ptr, ctx_ptr) };
> +
> +            to_result(ret)?;
> +        }
> +        LockKind::Try => {
> +            // SAFETY: `Mutex` is always pinned. If `AcquireCtx` is `Some`, it is pinned,
> +            // if `None`, it is set to `core::ptr::null_mut()`. Both cases are safe.
> +            let ret = unsafe { bindings::ww_mutex_trylock(mutex_ptr, ctx_ptr) };
> +
> +            if ret == 0 {
> +                return Err(EBUSY);
> +            } else {
> +                to_result(ret)?;
> +            }
> +        }
> +    };
> +
> +    Ok(MutexGuard::new(mutex))
> +}
>
> [snip]
>
> diff --git a/rust/kernel/sync/lock/ww_mutex/acquire_ctx.rs b/rust/kernel/sync/lock/ww_mutex/acquire_ctx.rs
> new file mode 100644
> index 0000000000000..18f4654e7df26
> --- /dev/null
> +++ b/rust/kernel/sync/lock/ww_mutex/acquire_ctx.rs
> @@ -0,0 +1,149 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Provides [`AcquireCtx`] for managing multiple wound/wait
> +//! mutexes from the same [`Class`].
> +
> +use super::{
> +    lock_common,
> +    Class,
> +    LockKind,
> +    Mutex,
> +    MutexGuard, //
> +};
> +use crate::{
> +    bindings,
> +    prelude::*,
> +    types::Opaque, //
> +};
> +use core::marker::PhantomData;
> +
> +/// Groups multiple [`Mutex`]es for deadlock avoidance when acquired
> +/// with the same [`Class`].
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use kernel::{
> +///     define_ww_class,
> +///     sync::{
> +///         lock::ww_mutex::{
> +///             AcquireCtx,
> +///             Class,
> +///             Mutex, //
> +///         },
> +///         Arc,
> +///     },
> +/// };
> +/// use pin_init::stack_pin_init;
> +///
> +/// define_ww_class!(SOME_WW_CLASS);
> +///
> +/// // Create mutexes.
> +/// let mutex1 = Arc::pin_init(Mutex::new(1, &SOME_WW_CLASS), GFP_KERNEL)?;
> +/// let mutex2 = Arc::pin_init(Mutex::new(2, &SOME_WW_CLASS), GFP_KERNEL)?;
> +///
> +/// // Create acquire context for deadlock avoidance.
> +/// let ctx = KBox::pin_init(AcquireCtx::new(&SOME_WW_CLASS), GFP_KERNEL)?;
> +///
> +/// // SAFETY: The guard is dropped before `ctx`.
> +/// let guard1 = unsafe { ctx.lock(&mutex1) }?;
> +/// // SAFETY: The guard is dropped before `ctx`.
> +/// let guard2 = unsafe { ctx.lock(&mutex2) }?;
> +///
> +/// // Mark acquisition phase as complete.
> +/// ctx.done();
> +///
> +/// # Ok::<(), Error>(())
> +/// ```
> +#[pin_data(PinnedDrop)]
> +#[repr(transparent)]
> +pub struct AcquireCtx<'a> {
> +    #[pin]
> +    pub(super) inner: Opaque<bindings::ww_acquire_ctx>,
> +    _p: PhantomData<&'a Class>,
> +}
> +
> +impl<'class> AcquireCtx<'class> {
> +    /// Initializes a new [`AcquireCtx`] with the given [`Class`].
> +    pub fn new(class: &'class Class) -> impl PinInit<Self> {
> +        let class_ptr = class.inner.get();
> +        pin_init!(AcquireCtx {
> +            inner <- Opaque::ffi_init(|slot: *mut bindings::ww_acquire_ctx| {
> +                // SAFETY: `class` is valid for the lifetime `'class` captured
> +                // by `AcquireCtx`.
> +                unsafe { bindings::ww_acquire_init(slot, class_ptr) }
> +            }),
> +            _p: PhantomData
> +        })
> +    }

I wonder if a better API would be

    class.new_context()

vs.

    AcquireCtx::new(&class)

?

> +
> +    /// Creates a [`AcquireCtx`] from a raw pointer.
> +    ///
> +    /// This function is intended for interoperability with C code.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller must ensure that `ptr` is a valid pointer to the `inner` field
> +    /// of [`AcquireCtx`] and that it remains valid for the lifetime `'a`.
> +    pub unsafe fn from_raw<'a>(ptr: *mut bindings::ww_acquire_ctx) -> &'a Self {
> +        // SAFETY: By the safety contract, `ptr` is valid to construct `AcquireCtx`.
> +        unsafe { &*ptr.cast() }
> +    }
> +
> +    /// Marks the end of the acquire phase.
> +    ///
> +    /// Calling this function is optional. It is just useful to document
> +    /// the code and clearly designated the acquire phase from actually
> +    /// using the locked data structures.
> +    ///
> +    /// After calling this function, no more mutexes can be acquired with
> +    /// this context.
> +    #[inline]
> +    pub fn done(&self) {
> +        // SAFETY: `self.inner` contains a valid, initialized acquire context.
> +        unsafe { bindings::ww_acquire_done(self.inner.get()) };
> +    }
> +
> +    /// Locks the given [`Mutex`] on this [`AcquireCtx`].
> +    ///
> +    /// # Safety
> +    ///
> +    /// On success, the caller must keep this context valid until the mutex is unlocked,
> +    /// even if the returned guard is forgotten. The context must not be finalized or
> +    /// reinitialized while the mutex remains locked.
> +    pub unsafe fn lock<'a, T>(&'a self, mutex: &'a Mutex<'a, T>) -> Result<MutexGuard<'a, T>> {
> +        lock_common(mutex, Some(self), LockKind::Regular)
> +    }
> +
> +    /// Similar to [`Self::lock`], but can be interrupted by signals.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The same requirements as [`Self::lock`] apply.
> +    pub unsafe fn lock_interruptible<'a, T>(
> +        &'a self,
> +        mutex: &'a Mutex<'a, T>,
> +    ) -> Result<MutexGuard<'a, T>> {
> +        lock_common(mutex, Some(self), LockKind::Interruptible)
> +    }
> +
> +    /// Tries to lock the [`Mutex`] on this [`AcquireCtx`] without blocking.
> +    ///
> +    /// Unlike [`Self::lock`], no deadlock handling is performed.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The same requirements as [`Self::lock`] apply.
> +    pub unsafe fn try_lock<'a, T>(&'a self, mutex: &'a Mutex<'a, T>) -> Result<MutexGuard<'a, T>> {
> +        lock_common(mutex, Some(self), LockKind::Try)
> +    }
> +}
> +
> +#[pinned_drop]
> +impl PinnedDrop for AcquireCtx<'_> {
> +    fn drop(self: Pin<&mut Self>) {

I wonder if we can just do

    let acquired = unsafe { (*self.inner.get()).acquired };
    assert!(!acquired);

to get rid of the `unsafe`. Somewhat similar to what we do with srcu. Of course,
ultimately we want to use `!Leak` for this and can remove the check.

Thoughts?

Best,
Gary

> +        // SAFETY: The locking methods require callers to release all acquired locks
> +        // before this context is dropped.
> +        unsafe { bindings::ww_acquire_fini(self.inner.get()) };
> +    }
> +}