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charset=utf-8 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3826.700.81\)) Subject: Re: [PATCH v7 4/6] rust: ww_mutex: add Mutex, AcquireCtx and MutexGuard From: Daniel Almeida In-Reply-To: <20251101161056.22408-5-work@onurozkan.dev> Date: Tue, 25 Nov 2025 15:32:08 -0300 Cc: rust-for-linux@vger.kernel.org, lossin@kernel.org, lyude@redhat.com, ojeda@kernel.org, alex.gaynor@gmail.com, boqun.feng@gmail.com, gary@garyguo.net, a.hindborg@kernel.org, aliceryhl@google.com, tmgross@umich.edu, dakr@kernel.org, peterz@infradead.org, mingo@redhat.com, will@kernel.org, longman@redhat.com, felipe_life@live.com, daniel@sedlak.dev, bjorn3_gh@protonmail.com, linux-kernel@vger.kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: References: <20251101161056.22408-1-work@onurozkan.dev> <20251101161056.22408-5-work@onurozkan.dev> To: =?utf-8?Q?Onur_=C3=96zkan?= X-Mailer: Apple Mail (2.3826.700.81) X-ZohoMailClient: External > On 1 Nov 2025, at 13:10, Onur =C3=96zkan wrote: >=20 > Implements full locking API (lock, try_lock, slow path, > interruptible variants) and integration with kernel bindings. >=20 > Signed-off-by: Onur =C3=96zkan > --- > rust/kernel/sync/lock/ww_mutex.rs | 276 ++++++++++++++++++ > rust/kernel/sync/lock/ww_mutex/acquire_ctx.rs | 211 +++++++++++++ > 2 files changed, 487 insertions(+) > create mode 100644 rust/kernel/sync/lock/ww_mutex/acquire_ctx.rs >=20 > diff --git a/rust/kernel/sync/lock/ww_mutex.rs = b/rust/kernel/sync/lock/ww_mutex.rs > index 727c51cc73af..2a9c1c20281b 100644 > --- a/rust/kernel/sync/lock/ww_mutex.rs > +++ b/rust/kernel/sync/lock/ww_mutex.rs > @@ -1,7 +1,283 @@ > // SPDX-License-Identifier: GPL-2.0 >=20 > //! 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. >=20 > +use crate::error::to_result; > +use crate::prelude::*; > +use crate::types::{NotThreadSafe, Opaque}; > +use crate::{bindings, container_of}; > + > +use core::cell::UnsafeCell; > +use core::marker::PhantomData; > + > +pub use acquire_ctx::AcquireCtx; > pub use class::Class; >=20 > +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::c_str; > +/// use kernel::sync::Arc; > +/// use kernel::sync::lock::ww_mutex::{AcquireCtx, Class, Mutex}; > +/// use pin_init::stack_pin_init; > +/// > +/// stack_pin_init!(let class =3D = Class::new_wound_wait(c_str!("some_class"))); > +/// let mutex =3D Arc::pin_init(Mutex::new(42, &class), GFP_KERNEL)?; > +/// > +/// let ctx =3D KBox::pin_init(AcquireCtx::new(&class), GFP_KERNEL)?; > +/// > +/// // SAFETY: Both `ctx` and `mutex` uses the same class. > +/// let guard =3D unsafe { ctx.lock(&mutex)? }; > +/// assert_eq!(*guard, 42); > +/// > +/// # Ok::<(), Error>(()) > +/// ``` > +#[pin_data] > +pub struct Mutex<'a, T: ?Sized> { > + #[pin] > + inner: Opaque, > + _p: PhantomData<&'a Class>, > + data: UnsafeCell, > +} > + > +// SAFETY: `Mutex` can be sent to another thread if the protected > +// data `T` can be. > +unsafe impl Send for Mutex<'_, T> {} > + > +// SAFETY: `Mutex` can be shared across threads if the protected > +// data `T` can be. > +unsafe impl Sync for Mutex<'_, 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 { > + let class_ptr =3D 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> 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` > + /// and that it remains valid for the lifetime `'a`. > + 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` > + // and remains valid for the lifetime `'a`. > + unsafe { &*container_of!(Opaque::cast_from(ptr), Self, inner) = } > + } > +} > + > +impl<'class, T: ?Sized> Mutex<'class, T> { > + /// Checks if the mutex is currently locked. > + pub fn is_locked(&self) -> bool { > + // SAFETY: The mutex is pinned and valid. > + unsafe { bindings::ww_mutex_is_locked(self.inner.get()) } > + } > + > + /// Locks the given mutex without acquire context = ([`AcquireCtx`]). > + pub fn lock<'a>(&'a self) -> Result> { > + // SAFETY: `ctx` is `None`, so no class matching is required. > + unsafe { lock_common(self, None, LockKind::Regular) } > + } > + > + /// Similar to `lock`, but can be interrupted by signals. > + pub fn lock_interruptible<'a>(&'a self) -> Result> { > + // SAFETY: `ctx` is `None`, so no class matching is required. > + unsafe { lock_common(self, None, LockKind::Interruptible) } > + } > + > + /// Locks the given mutex without acquire context = ([`AcquireCtx`]) using the slow path. > + /// > + /// This function should be used when `lock` fails (typically due = to a potential deadlock). > + pub fn lock_slow<'a>(&'a self) -> Result> { > + // SAFETY: `ctx` is `None`, so no class matching is required. > + unsafe { lock_common(self, None, LockKind::Slow) } > + } > + > + /// Similar to `lock_slow`, but can be interrupted by signals. > + pub fn lock_slow_interruptible<'a>(&'a self) -> = Result> { > + // SAFETY: `ctx` is `None`, so no class matching is required. > + unsafe { lock_common(self, None, LockKind::SlowInterruptible) = } > + } > + > + /// Tries to lock the mutex with no [`AcquireCtx`] and without = blocking. > + /// > + /// Unlike `lock`, no deadlock handling is performed. > + pub fn try_lock<'a>(&'a self) -> Result> { > + // SAFETY: `ctx` is `None`, so no class matching is required. > + unsafe { lock_common(self, None, LockKind::Try) } > + } > +} > + > +/// 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 =3D "the lock unlocks immediately when the guard is = unused"] > +pub struct MutexGuard<'a, T: ?Sized> { > + mutex: &'a Mutex<'a, T>, > + _not_send: NotThreadSafe, > +} > + > +// SAFETY: [`MutexGuard`] can be shared between threads if the data = can. > +unsafe impl Sync for MutexGuard<'_, T> {} > + > +impl<'a, T: ?Sized> MutexGuard<'a, T> { > + /// Creates a new guard for a locked mutex. > + fn new(mutex: &'a Mutex<'a, T>) -> Self { > + Self { > + mutex, > + _not_send: NotThreadSafe, > + } > + } > +} > + > +impl<'a> MutexGuard<'a, ()> { > + /// Creates a [`MutexGuard`] 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` > + /// and that it remains valid for the lifetime `'a`. > + pub unsafe fn from_raw<'b>(ptr: *mut bindings::ww_mutex) -> = MutexGuard<'b, ()> { > + // SAFETY: By the safety contract, the caller guarantees that = `ptr` > + // points to a valid `ww_mutex` which is the `mutex` field of = `Mutex` > + // and remains valid for the lifetime `'a`. > + let mutex =3D unsafe { Mutex::from_raw(ptr) }; > + > + MutexGuard::new(mutex) > + } > +} > + > +impl core::ops::Deref for MutexGuard<'_, T> { > + type Target =3D T; > + > + fn deref(&self) -> &Self::Target { > + // SAFETY: We hold the lock, so we have exclusive access. > + unsafe { &*self.mutex.data.get() } > + } > +} > + > +impl core::ops::DerefMut for MutexGuard<'_, 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() } > + } > +} > + > +impl Drop for MutexGuard<'_, T> { > + fn drop(&mut self) { > + // SAFETY: We hold the lock and are about to release it. > + unsafe { bindings::ww_mutex_unlock(self.mutex.inner.get()) }; > + } > +} > + > +/// 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, > + /// Used in slow path after deadlock detection. > + Slow, > + /// Slow path but interruptible. > + SlowInterruptible, > + /// Does not block, returns immediately if busy. > + Try, > +} > + > +/// Internal helper that unifies the different locking kinds. > +/// > +/// # Safety > +/// > +/// If `ctx` is `Some`, the given `mutex` must be created with the = [`Class`] that > +/// was used to initialize `ctx`. > +unsafe fn lock_common<'a, T: ?Sized>( > + mutex: &'a Mutex<'a, T>, > + ctx: Option<&AcquireCtx<'_>>, > + kind: LockKind, > +) -> Result> { > + let ctx_ptr =3D ctx.map_or(core::ptr::null_mut(), |c| = c.inner.get()); > + > + let mutex_ptr =3D mutex.inner.get(); > + > + match kind { > + LockKind::Regular =3D> { > + // 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 =3D unsafe { bindings::ww_mutex_lock(mutex_ptr, = ctx_ptr) }; > + > + to_result(ret)?; > + } > + LockKind::Interruptible =3D> { > + // 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 =3D unsafe { = bindings::ww_mutex_lock_interruptible(mutex_ptr, ctx_ptr) }; > + > + to_result(ret)?; > + } > + LockKind::Slow =3D> { > + // 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. > + unsafe { bindings::ww_mutex_lock_slow(mutex_ptr, ctx_ptr) = }; > + } > + LockKind::SlowInterruptible =3D> { > + // 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 =3D unsafe { = bindings::ww_mutex_lock_slow_interruptible(mutex_ptr, ctx_ptr) }; > + > + to_result(ret)?; > + } > + LockKind::Try =3D> { > + // 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 =3D unsafe { = bindings::ww_mutex_trylock(mutex_ptr, ctx_ptr) }; > + > + if ret =3D=3D 0 { > + return Err(EBUSY); > + } else { > + to_result(ret)?; > + } > + } > + }; > + > + Ok(MutexGuard::new(mutex)) > +} > 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 000000000000..65a955b5809d > --- /dev/null > +++ b/rust/kernel/sync/lock/ww_mutex/acquire_ctx.rs > @@ -0,0 +1,211 @@ > +// SPDX-License-Identifier: GPL-2.0 > + > +//! Provides [`AcquireCtx`] for managing multiple wound/wait > +//! mutexes from the same [`Class`]. > + > +use crate::bindings; > +use crate::prelude::*; > +use crate::types::Opaque; > + > +use core::marker::PhantomData; > + > +use super::{lock_common, Class, LockKind, Mutex, MutexGuard}; > + > +/// Groups multiple [`Mutex`]es for deadlock avoidance when acquired > +/// with the same [`Class`]. > +/// > +/// # Examples > +/// > +/// ``` > +/// use kernel::sync::lock::ww_mutex::{Class, AcquireCtx, Mutex}; > +/// use kernel::c_str; > +/// use kernel::sync::Arc; > +/// use pin_init::stack_pin_init; > +/// > +/// stack_pin_init!(let class =3D = Class::new_wound_wait(c_str!("demo"))); > +/// > +/// // Create mutexes. > +/// let mutex1 =3D Arc::pin_init(Mutex::new(1, &class), GFP_KERNEL)?; > +/// let mutex2 =3D Arc::pin_init(Mutex::new(2, &class), GFP_KERNEL)?; > +/// > +/// // Create acquire context for deadlock avoidance. > +/// let ctx =3D KBox::pin_init(AcquireCtx::new(&class), GFP_KERNEL)?; > +/// > +/// let guard1 =3D unsafe { ctx.lock(&mutex1)? }; > +/// let guard2 =3D unsafe { ctx.lock(&mutex2)? }; > +/// > +/// // Mark acquisition phase as complete. > +/// // SAFETY: It's called exactly once here and nowhere else. > +/// unsafe { ctx.done() }; > +/// > +/// # Ok::<(), Error>(()) > +/// ``` > +#[pin_data(PinnedDrop)] > +#[repr(transparent)] > +pub struct AcquireCtx<'a> { > + #[pin] > + pub(super) inner: Opaque, > + _p: PhantomData<&'a Class>, > +} > + > +impl<'class> AcquireCtx<'class> { > + /// Initializes a new [`AcquireCtx`] with the given `class`. > + pub fn new(class: &'class Class) -> impl PinInit { > + let class_ptr =3D 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 > + }) > + } > + > + /// 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. > + /// > + /// # Safety > + /// > + /// The caller must ensure that this function is called only = once. > + pub unsafe fn done(&self) { > + // SAFETY: By the safety contract, the caller guarantees that = this > + // function is called only once. > + unsafe { bindings::ww_acquire_done(self.inner.get()) }; > + } > + > + /// Re-initializes the [`AcquireCtx`]. > + /// > + /// Must be called after releasing all locks when [`EDEADLK`] = occurs. > + /// > + /// # Safety > + /// > + /// The given class must be equal to the class that was used to > + /// initialize this [`AcquireCtx`]. > + pub unsafe fn reinit(self: Pin<&mut Self>, class: &'class Class) = { > + let ctx =3D self.inner.get(); > + > + // SAFETY: > + // - Lifetime of any guard (which hold an immutable borrow = of `self`) cannot overlap > + // with the execution of this function. This enforces that = all locks acquired via > + // this context have been released. > + // > + // - `ctx` is valid pointer to a `ww_acquire_ctx`. > + // > + // - `ctx` is guaranteed to be initialized because = `ww_acquire_fini` > + // can only be called from the `Drop` implementation. > + // > + // - `ww_acquire_fini` is safe to call on an initialized = context. > + unsafe { bindings::ww_acquire_fini(ctx) }; > + > + // SAFETY: > + // - `ctx` is valid pointer to a `ww_acquire_ctx`. > + // > + // - `class` is a valid pointer to a `ww_class`. > + // > + // - `ww_acquire_init` is safe to call with valid pointers > + // to initialize an uninitialized context. > + // > + // - By the safety contract, the caller guarantees that the = given > + // `class` is the same as the one used to initialize this = `AcquireCtx`. > + unsafe { bindings::ww_acquire_init(ctx, class.inner.get()) }; > + } > + > + /// Locks the given mutex on this [`AcquireCtx`]. > + /// > + /// # Safety > + /// > + /// The given `mutex` must be created with the [`Class`] that was = used > + /// to initialize this [`AcquireCtx`]. > + pub unsafe fn lock<'a, T>(&'a self, mutex: &'a Mutex<'a, T>) -> = Result> { > + // SAFETY: By the safety contract, `mutex` belongs to the = same `Class` > + // as `self` does. > + unsafe { lock_common(mutex, Some(self), LockKind::Regular) } > + } > + > + /// Similar to `lock`, but can be interrupted by signals. > + /// > + /// # Safety > + /// > + /// The given `mutex` must be created with the [`Class`] that was = used > + /// to initialize this [`AcquireCtx`]. > + pub unsafe fn lock_interruptible<'a, T>( > + &'a self, > + mutex: &'a Mutex<'a, T>, > + ) -> Result> { > + // SAFETY: By the safety contract, `mutex` belongs to the = same `Class` > + // as `self` does. > + unsafe { lock_common(mutex, Some(self), = LockKind::Interruptible) } > + } > + > + /// Locks the given mutex on this [`AcquireCtx`] using the slow = path. > + /// > + /// This function should be used when `lock` fails (typically due = to a potential deadlock). > + /// > + /// # Safety > + /// > + /// The given `mutex` must be created with the [`Class`] that was = used > + /// to initialize this [`AcquireCtx`]. I=E2=80=99m assuming we either can=E2=80=99t or otherwise don=E2=80=99t = want to store this in the Mutex itself? IOW: the class is =E2=80=99static IIUC, so it=E2=80=99s always safe to = store a reference anyways, at the cost of providing the storage. > + pub unsafe fn lock_slow<'a, T>(&'a self, mutex: &'a Mutex<'a, T>) = -> Result> { > + // SAFETY: By the safety contract, `mutex` belongs to the = same `Class` > + // as `self` does. > + unsafe { lock_common(mutex, Some(self), LockKind::Slow) } > + } > + > + /// Similar to `lock_slow`, but can be interrupted by signals. > + /// > + /// # Safety > + /// > + /// The given `mutex` must be created with the [`Class`] that was = used > + /// to initialize this [`AcquireCtx`]. > + pub unsafe fn lock_slow_interruptible<'a, T>( > + &'a self, > + mutex: &'a Mutex<'a, T>, > + ) -> Result> { > + // SAFETY: By the safety contract, `mutex` belongs to the = same `Class` > + // as `self` does. > + unsafe { lock_common(mutex, Some(self), = LockKind::SlowInterruptible) } > + } > + > + /// Tries to lock the mutex on this [`AcquireCtx`] without = blocking. > + /// > + /// Unlike `lock`, no deadlock handling is performed. You can use [`lock`] here for better docs. > + /// > + /// # Safety > + /// > + /// The given `mutex` must be created with the [`Class`] that was = used > + /// to initialize this [`AcquireCtx`]. > + pub unsafe fn try_lock<'a, T>(&'a self, mutex: &'a Mutex<'a, T>) = -> Result> { > + // SAFETY: By the safety contract, `mutex` belongs to the = same `Class` > + // as `self` does. > + unsafe { lock_common(mutex, Some(self), LockKind::Try) } > + } > +} > + > +#[pinned_drop] > +impl PinnedDrop for AcquireCtx<'_> { > + fn drop(self: Pin<&mut Self>) { > + // SAFETY: Given the lifetime bounds we know no locks are = held, > + // so calling `ww_acquire_fini` is safe. > + unsafe { bindings::ww_acquire_fini(self.inner.get()) }; > + } > +} > --=20 > 2.51.2 >=20 >=20 =E2=80=94 Daniel=