Re: [PATCH v6 3/7] rust: implement `WwMutex`, `WwAcquireCtx` and `WwMutexGuard`

Onur
2025-09-06T11:35:58Z

On Fri, 5 Sep 2025 15:49:57 -0300
Daniel Almeida <daniel.almeida@collabora.com> wrote:

> Hi Onur,
>
> I think this is starting to come together IMHO. Some comments inline.
>
> > On 3 Sep 2025, at 10:13, Onur Özkan <work@onurozkan.dev> wrote:
> >
> > Includes full locking API (lock, try_lock, slow path, interruptible
> > variants) and integration with kernel bindings.
> >
> > Also adds the `EDEADLK` error code to support deadlock detection.
> >
> > Signed-off-by: Onur Özkan <work@onurozkan.dev>
> > ---
> > rust/kernel/error.rs | 1 +
> > rust/kernel/sync/lock/ww_mutex.rs | 289
> > +++++++++++++++++++++++++++++- 2 files changed, 289 insertions(+),
> > 1 deletion(-)
> >
> > diff --git a/rust/kernel/error.rs b/rust/kernel/error.rs
> > index a41de293dcd1..560de6117094 100644
> > --- a/rust/kernel/error.rs
> > +++ b/rust/kernel/error.rs
> > @@ -64,6 +64,7 @@ macro_rules! declare_err {
> > declare_err!(EPIPE, "Broken pipe.");
> > declare_err!(EDOM, "Math argument out of domain of func.");
> > declare_err!(ERANGE, "Math result not representable.");
> > + declare_err!(EDEADLK, "Resource deadlock avoided.");
> > declare_err!(EOVERFLOW, "Value too large for defined data
> > type."); declare_err!(ETIMEDOUT, "Connection timed out.");
> > declare_err!(ERESTARTSYS, "Restart the system call.");
> > diff --git a/rust/kernel/sync/lock/ww_mutex.rs
> > b/rust/kernel/sync/lock/ww_mutex.rs index
> > ca5b4525ace6..314360632953 100644 ---
> > a/rust/kernel/sync/lock/ww_mutex.rs +++
> > b/rust/kernel/sync/lock/ww_mutex.rs @@ -10,8 +10,11 @@
> > //! For more information:
> > <https://docs.kernel.org/locking/ww-mutex-design.html>
> >
> > use crate::bindings;
> > +use crate::error::to_result;
> > use crate::prelude::*;
> > -use crate::types::Opaque;
> > +use crate::types::{NotThreadSafe, Opaque};
> > +use core::cell::UnsafeCell;
> > +use core::marker::PhantomData;
> >
> > /// Create static [`WwClass`] instances.
> > ///
> > @@ -134,3 +137,287 @@ pub fn new_wound_wait(name: &'static CStr) ->
> > impl PinInit<Self> { Self::new(name, false)
> > }
> > }
> > +
> > +/// Groups multiple mutex acquisitions together for deadlock
> > avoidance. +///
> > +/// Must be used when acquiring multiple mutexes of the same class.
> > +///
> > +/// # Examples
> > +///
> > +/// ```
> > +/// use kernel::sync::lock::ww_mutex::{WwClass, WwAcquireCtx,
> > WwMutex}; +/// use kernel::c_str;
> > +/// use kernel::sync::Arc;
> > +/// use pin_init::stack_pin_init;
> > +///
> > +/// stack_pin_init!(let class =
> > WwClass::new_wound_wait(c_str!("my_class"))); +///
> > +/// // Create mutexes.
> > +/// let mutex1 = Arc::pin_init(WwMutex::new(1, &class),
> > GFP_KERNEL)?; +/// let mutex2 = Arc::pin_init(WwMutex::new(2,
> > &class), GFP_KERNEL)?; +///
> > +/// // Create acquire context for deadlock avoidance.
> > +/// let ctx = KBox::pin_init(WwAcquireCtx::new(&class),
> > GFP_KERNEL)?; +///
> > +/// // Acquire multiple locks safely.
> > +/// let guard1 = ctx.lock(&mutex1)?;
> > +/// let guard2 = ctx.lock(&mutex2)?;
> > +///
> > +/// // Mark acquisition phase as complete.
> > +/// ctx.done();
> > +///
> > +/// # Ok::<(), Error>(())
> > +/// ```
> > +#[pin_data(PinnedDrop)]
> > +pub struct WwAcquireCtx<'a> {
>
> Please drop the Ww prefix. This is ww_mutex.rs <http://ww_mutex.rs/>
> after all.
>
> > + #[pin]
> > + inner: Opaque<bindings::ww_acquire_ctx>,
> > + _p: PhantomData<&'a WwClass>,
> > +}
> > +
> > +impl<'ww_class> WwAcquireCtx<'ww_class> {
> > + /// Initializes `Self` with calling C side `ww_acquire_init`
> > inside.
> > + pub fn new(ww_class: &'ww_class WwClass) -> impl PinInit<Self>
> > {
> > + let class = ww_class.inner.get();
> > + pin_init!(WwAcquireCtx {
> > + inner <- Opaque::ffi_init(|slot: *mut
> > bindings::ww_acquire_ctx| {
> > + // SAFETY: `ww_class` is valid for the lifetime
> > `'ww_class` captured by `Self`.
> > + unsafe { bindings::ww_acquire_init(slot, class) }
> > + }),
> > + _p: PhantomData
> > + })
> > + }
> > +
> > + /// Marks the end of the acquire phase.
> > + ///
> > + /// After calling this function, no more mutexes can be
> > acquired with this context.
> > + pub fn done(&self) {
> > + // SAFETY: The context is pinned and valid.
> > + unsafe { bindings::ww_acquire_done(self.inner.get()) };
> > + }
>
> This lets you call done() multiple times. We should probably use a
> typestate here.
>

Good point. I will see what I can do.

> > +
> > + /// Locks the given mutex.
> > + pub fn lock<'a, T>(&'a self, ww_mutex: &'a WwMutex<'a, T>) ->
> > Result<WwMutexGuard<'a, T>> {
>
> > + // SAFETY: The mutex is pinned and valid.
> > + let ret = unsafe {
> > bindings::ww_mutex_lock(ww_mutex.mutex.get(), self.inner.get()) }; +
> > + to_result(ret)?;
> > +
> > + Ok(WwMutexGuard::new(ww_mutex))
> > + }
> > +
> > + /// Similar to `lock`, but can be interrupted by signals.
> > + pub fn lock_interruptible<'a, T>(
> > + &'a self,
> > + ww_mutex: &'a WwMutex<'a, T>,
> > + ) -> Result<WwMutexGuard<'a, T>> {
> > + // SAFETY: The mutex is pinned and valid.
> > + let ret = unsafe {
> > +
> > bindings::ww_mutex_lock_interruptible(ww_mutex.mutex.get(),
> > self.inner.get())
> > + };
> > +
> > + to_result(ret)?;
> > +
> > + Ok(WwMutexGuard::new(ww_mutex))
> > + }
> > +
> > + /// Locks the given mutex using the slow path.
> > + ///
> > + /// This function should be used when `lock` fails (typically
> > due to a potential deadlock).
> > + pub fn lock_slow<'a, T>(&'a self, ww_mutex: &'a WwMutex<'a,
> > T>) -> Result<WwMutexGuard<'a, T>> {
> > + // SAFETY: The mutex is pinned and valid, and we're in the
> > slow path.
> > + unsafe {
> > bindings::ww_mutex_lock_slow(ww_mutex.mutex.get(),
> > self.inner.get()) }; +
> > + Ok(WwMutexGuard::new(ww_mutex))
> > + }
> > +
> > + /// Similar to `lock_slow`, but can be interrupted by signals.
> > + pub fn lock_slow_interruptible<'a, T>(
> > + &'a self,
> > + ww_mutex: &'a WwMutex<'a, T>,
> > + ) -> Result<WwMutexGuard<'a, T>> {
> > + // SAFETY: The mutex is pinned and valid, and we are in
> > the slow path.
> > + let ret = unsafe {
> > +
> > bindings::ww_mutex_lock_slow_interruptible(ww_mutex.mutex.get(),
> > self.inner.get())
> > + };
> > +
> > + to_result(ret)?;
> > +
> > + Ok(WwMutexGuard::new(ww_mutex))
> > + }
> > +
> > + /// Tries to lock the mutex without blocking.
> > + ///
> > + /// Unlike `lock`, no deadlock handling is performed.
> > + pub fn try_lock<'a, T>(&'a self, ww_mutex: &'a WwMutex<'a, T>)
> > -> Result<WwMutexGuard<'a, T>> {
> > + // SAFETY: The mutex is pinned and valid.
> > + let ret = unsafe {
> > bindings::ww_mutex_trylock(ww_mutex.mutex.get(), self.inner.get())
> > }; +
> > + if ret == 0 {
> > + return Err(EBUSY);
> > + } else {
> > + to_result(ret)?;
> > + }
> > +
> > + Ok(WwMutexGuard::new(ww_mutex))
> > + }
> > +}
> > +
> > +#[pinned_drop]
> > +impl PinnedDrop for WwAcquireCtx<'_> {
> > + fn drop(self: Pin<&mut Self>) {
> > + // SAFETY: The context is being dropped and is pinned.
> > + unsafe { bindings::ww_acquire_fini(self.inner.get()) };
> > + }
> > +}
> > +
> > +/// A wound/wait mutex backed with C side `ww_mutex`.
> > +///
> > +/// This is a mutual exclusion primitive that provides deadlock
> > avoidance when +/// acquiring multiple locks of the same class.
>
> A link would be cool for the docs.
>
> > +///
> > +/// # Examples
> > +///
> > +/// ## Basic Usage
> > +///
> > +/// ```
> > +/// use kernel::c_str;
> > +/// use kernel::sync::Arc;
> > +/// use kernel::sync::lock::ww_mutex::{WwClass, WwAcquireCtx,
> > WwMutex }; +/// use pin_init::stack_pin_init;
> > +///
> > +/// stack_pin_init!(let class =
> > WwClass::new_wound_wait(c_str!("buffer_class"))); +/// let mutex =
> > Arc::pin_init(WwMutex::new(42, &class), GFP_KERNEL)?; +///
> > +/// let ctx = KBox::pin_init(WwAcquireCtx::new(&class),
> > GFP_KERNEL)?; +///
> > +/// let guard = ctx.lock(&mutex)?;
> > +/// assert_eq!(*guard, 42);
> > +///
> > +/// # Ok::<(), Error>(())
> > +/// ```
> > +///
> > +/// ## Multiple Locks
> > +///
> > +/// ```
> > +/// use kernel::c_str;
> > +/// use kernel::prelude::*;
> > +/// use kernel::sync::Arc;
> > +/// use kernel::sync::lock::ww_mutex::{WwClass, WwAcquireCtx,
> > WwMutex}; +/// use pin_init::stack_pin_init;
> > +///
> > +/// stack_pin_init!(let class =
> > WwClass::new_wait_die(c_str!("resource_class"))); +/// let mutex_a
> > = Arc::pin_init(WwMutex::new("Resource A", &class), GFP_KERNEL)?;
> > +/// let mutex_b = Arc::pin_init(WwMutex::new("Resource B",
> > &class), GFP_KERNEL)?; +/// +/// let ctx =
> > KBox::pin_init(WwAcquireCtx::new(&class), GFP_KERNEL)?; +///
> > +/// // Try to acquire both locks.
> > +/// let guard_a = match ctx.lock(&mutex_a) {
> > +/// Ok(guard) => guard,
> > +/// Err(e) if e == EDEADLK => {
> > +/// // Deadlock detected, use slow path.
>
> You must release all other locks before calling this, except there
> aren’t any taken in your example.
>

True, thanks!

> You should perhaps add a release_all() function to the context.
>

Do you mean we should track guards in `WwAcquireCtx` wrapper? That
would drop the need for `ExecContext` (mostly), which is a good idea
IMO.

> > +/// ctx.lock_slow(&mutex_a)?
> > +/// }
> > +/// Err(e) => return Err(e),
> > +/// };
> > +///
> > +/// let guard_b = ctx.lock(&mutex_b)?;
> > +/// ctx.done();
> > +///
> > +/// # Ok::<(), Error>(())
> > +/// ```
> > +#[pin_data]
> > +pub struct WwMutex<'a, T: ?Sized> {
> > + _p: PhantomData<&'a WwClass>,
>
> Make the PhantomData last, please.
>
> > + #[pin]
> > + mutex: Opaque<bindings::ww_mutex>,
> > + data: UnsafeCell<T>,
> > +}
> > +
> > +// SAFETY: [`WwMutex`] can be shared between threads.
> > +unsafe impl<T: ?Sized + Send> Send for WwMutex<'_, T> {}
>
> “Send” does not share anything. When you send something, some other
> thread has it, and you don’t have it anymore.
>
> Blank here.
>
> > +// SAFETY: [`WwMutex`] can be safely accessed from multiple
> > threads concurrently. +unsafe impl<T: ?Sized + Send + Sync> Sync
> > for WwMutex<'_, T> {} +
> > +impl<'ww_class, T> WwMutex<'ww_class, T> {
> > + /// Creates `Self` with calling `ww_mutex_init` inside.
>
> ^ This does not parse very well.
>
> > + pub fn new(t: T, ww_class: &'ww_class WwClass) -> impl
> > PinInit<Self> {
>
> Please rename “t” to “data”.
>
> > + let class = ww_class.inner.get();
> > + pin_init!(WwMutex {
> > + mutex <- Opaque::ffi_init(|slot: *mut
> > bindings::ww_mutex| {
> > + // SAFETY: `ww_class` is valid for the lifetime
> > `'ww_class` captured by `Self`.
> > + unsafe { bindings::ww_mutex_init(slot, class) }
> > + }),
> > + data: UnsafeCell::new(t),
> > + _p: PhantomData,
> > + })
> > + }
> > +}
> > +
> > +impl<T: ?Sized> WwMutex<'_, T> {
>
> I wonder why we need this ?Sized here?
>

If I recall it correctly it was required by `self.mutex.get()` call. I
will re-check when I am working for the next patch.

> > + /// Returns a raw pointer to the inner mutex.
> > + fn as_ptr(&self) -> *mut bindings::ww_mutex {
> > + self.mutex.get()
> > + }
> > +
> > + /// Checks if the mutex is currently locked.
> > + ///
> > + /// Intended for internal tests only and should not be used
> > + /// anywhere else.
>
> Why?
>

It was requested by Boqun and Peter here [0].

[0]:
https://lore.kernel.org/rust-for-linux/aFReIdlPPg4MmaYX@tardis.local/

> > + #[cfg(CONFIG_KUNIT)]
> > + fn is_locked(&self) -> bool {
>
> I’d recommend removing this CONFIG_KUNIT and making this pub. You can
> see that there are users for this function in the C code, like for
> example, dma_resv_is_locked().
>
> > + // SAFETY: The mutex is pinned and valid.
> > + unsafe { bindings::ww_mutex_is_locked(self.mutex.get()) }
> > + }
> > +}
> > +
> > +/// A guard that provides exclusive access to the data protected
> > +/// by a [`WwMutex`].
> > +///
> > +/// # Invariants
> > +///
> > +/// The guard holds an exclusive lock on the associated
> > [`WwMutex`]. 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 WwMutexGuard<'a, T: ?Sized> {
> > + mutex: &'a WwMutex<'a, T>,
> > + _not_send: NotThreadSafe,
> > +}
> > +
> > +// SAFETY: [`WwMutexGuard`] can be shared between threads if the
> > data can. +unsafe impl<T: ?Sized + Sync> Sync for WwMutexGuard<'_,
> > T> {} +
> > +impl<'a, T: ?Sized> WwMutexGuard<'a, T> {
> > + /// Creates a new guard for a locked mutex.
> > + fn new(mutex: &'a WwMutex<'a, T>) -> Self {
> > + Self {
> > + mutex,
> > + _not_send: NotThreadSafe,
> > + }
> > + }
> > +}
> > +
> > +impl<T: ?Sized> core::ops::Deref for WwMutexGuard<'_, T> {
> > + type Target = T;
> > +
> > + fn deref(&self) -> &Self::Target {
> > + // SAFETY: We hold the lock, so we have exclusive access.
> > + unsafe { &*self.mutex.data.get() }
> > + }
> > +}
> > +
> > +impl<T: ?Sized> core::ops::DerefMut for WwMutexGuard<'_, T> {
> > + fn deref_mut(&mut self) -> &mut Self::Target {
> > + // SAFETY: We hold the lock, so we have exclusive access.
> > + unsafe { &mut *self.mutex.data.get() }
> > + }
>
> We need to add a bound on Unpin. See [0].
>
> > +}
> > +
> > +impl<T: ?Sized> Drop for WwMutexGuard<'_, T> {
> > + fn drop(&mut self) {
> > + // SAFETY: We hold the lock and are about to release it.
> > + unsafe { bindings::ww_mutex_unlock(self.mutex.as_ptr()) };
> > + }
> > +}
> > —
> > 2.50.0
> >
> >
>
> [0]:
> https://lore.kernel.org/rust-for-linux/20250828-lock-t-when-t-is-pinned-v2-1-b067c4b93fd6@collabora.com/
>