Re: [PATCH v12 3/6] rust: implement Class for ww_class support

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

On Mon Oct 12, 2026 at 11:26 AM BST, Onur Özkan wrote:
> Adds the Class type, the first step in supporting ww_mutex in Rust.
> Class represents ww_class, used for deadlock avoidance for supporting
> both wait-die and wound-wait semantics.
>
> Also adds the define_ww_class and define_wd_class macros for safely
> declaring static instances.
>
> Reviewed-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
> Signed-off-by: Onur Özkan <work@onurozkan.dev>

Reviewed-by: Gary Guo <gary@garyguo.net>

With some nits below.

> ---
>  rust/kernel/sync/lock.rs                |   1 +
>  rust/kernel/sync/lock/ww_mutex.rs       |   7 ++
>  rust/kernel/sync/lock/ww_mutex/class.rs | 151 ++++++++++++++++++++++++
>  3 files changed, 159 insertions(+)
>  create mode 100644 rust/kernel/sync/lock/ww_mutex.rs
>  create mode 100644 rust/kernel/sync/lock/ww_mutex/class.rs
>
> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> index 10b6b5e9b024f..f509250822c55 100644
> --- a/rust/kernel/sync/lock.rs
> +++ b/rust/kernel/sync/lock.rs
> @@ -15,6 +15,7 @@
>  
>  pub mod mutex;
>  pub mod spinlock;
> +pub mod ww_mutex;
>  
>  pub(super) mod global;
>  pub use global::{GlobalGuard, GlobalLock, GlobalLockBackend, GlobalLockedBy};
> diff --git a/rust/kernel/sync/lock/ww_mutex.rs b/rust/kernel/sync/lock/ww_mutex.rs
> new file mode 100644
> index 0000000000000..727c51cc73af8
> --- /dev/null
> +++ b/rust/kernel/sync/lock/ww_mutex.rs
> @@ -0,0 +1,7 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Rust abstractions for the kernel's wound-wait locking primitives.
> +
> +pub use class::Class;
> +
> +mod class;
> diff --git a/rust/kernel/sync/lock/ww_mutex/class.rs b/rust/kernel/sync/lock/ww_mutex/class.rs
> new file mode 100644
> index 0000000000000..58a4467552700
> --- /dev/null
> +++ b/rust/kernel/sync/lock/ww_mutex/class.rs
> @@ -0,0 +1,151 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Provides [`Class`] to group wound/wait mutexes to be acquired together
> +//! and specifies which deadlock avoidance algorithm to use (e.g., wound-wait
> +//! or wait-die).
> +//!
> +//! The [`define_ww_class!`] and [`define_wd_class!`] macros provide safe
> +//! ways to create classes.
> +
> +use crate::{
> +    bindings,
> +    prelude::*,
> +    types::Opaque, //
> +};
> +
> +/// Defines a static wound-wait [`Class`].
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use kernel::define_ww_class;
> +///
> +/// define_ww_class!(SOME_WW_CLASS);
> +/// ```
> +#[macro_export]
> +macro_rules! define_ww_class {
> +    ($name:ident) => {
> +        static $name: $crate::sync::lock::ww_mutex::Class =
> +            // SAFETY: This is `static`, so address is fixed and won't move.
> +            unsafe {
> +                $crate::sync::lock::ww_mutex::Class::new_unpinned(
> +                    $crate::c_str!(::core::concat!(::core::stringify!($name), "_acquire")),
> +                    $crate::c_str!(::core::concat!(::core::stringify!($name), "_mutex")),
> +                    false,
> +                )
> +            };
> +    };
> +}
> +
> +/// Defines a static wait-die [`Class`].
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use kernel::define_wd_class;
> +///
> +/// define_wd_class!(SOME_WD_CLASS);
> +/// ```
> +#[macro_export]
> +macro_rules! define_wd_class {
> +    ($name:ident) => {
> +        static $name: $crate::sync::lock::ww_mutex::Class =
> +            // SAFETY: This is `static`, so address is fixed and won't move.
> +            unsafe {
> +                $crate::sync::lock::ww_mutex::Class::new_unpinned(
> +                    $crate::c_str!(::core::concat!(::core::stringify!($name), "_acquire")),
> +                    $crate::c_str!(::core::concat!(::core::stringify!($name), "_mutex")),
> +                    true,
> +                )
> +            };
> +    };
> +}
> +
> +/// Used to group mutexes together for deadlock avoidance.
> +///
> +/// All mutexes that might be acquired together should use the same class.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// use kernel::{
> +///     define_wd_class,
> +///     define_ww_class, //
> +/// };
> +///
> +/// define_ww_class!(SOME_WW_CLASS);
> +/// define_wd_class!(SOME_WD_CLASS);
> +///
> +/// # Ok::<(), Error>(())
> +/// ```
> +#[pin_data]
> +#[repr(transparent)]
> +pub struct Class {
> +    #[pin]
> +    pub(super) inner: Opaque<bindings::ww_class>,
> +}
> +
> +impl Class {
> +    /// Creates an unpinned [`Class`].
> +    ///
> +    /// You should prefer using [`define_ww_class!`] and [`define_wd_class!`]
> +    /// macros. This function is `pub` only so that those macros can use it.
> +    /// The alternative would be to expose the private fields of [`Class`]
> +    /// which is less desirable.

Replace the paragraph with `#[doc(hidden)]`?

> +    ///
> +    /// # Safety
> +    ///
> +    /// Caller must guarantee that the returned value must be pinned before
> +    /// its first use.
> +    pub const unsafe fn new_unpinned(
> +        acquire_name: &'static CStr,
> +        mutex_name: &'static CStr,
> +        is_wait_die: bool,
> +    ) -> Self {
> +        Class {
> +            inner: Opaque::new(bindings::ww_class {
> +                stamp: bindings::atomic_long_t { counter: 0 },
> +                acquire_name: crate::str::as_char_ptr_in_const_context(acquire_name),
> +                mutex_name: crate::str::as_char_ptr_in_const_context(mutex_name),
> +                is_wait_die: is_wait_die as u32,
> +                // TODO: Replace with `bindings::lock_class_key::default()` once
> +                // stabilized for `const`.
> +                //
> +                // SAFETY: This is always zero-initialized when defined with
> +                // `DEFINE_WD_CLASS` globally on C side.
> +                //
> +                // For reference, see __WW_CLASS_INITIALIZER() in
> +                // "include/linux/ww_mutex.h".
> +                acquire_key: unsafe { core::mem::zeroed() },
> +                // TODO: Replace with `bindings::lock_class_key::default()` once
> +                // stabilized for `const`.
> +                //
> +                // SAFETY: This is always zero-initialized when defined with
> +                // `DEFINE_WD_CLASS` globally on C side.
> +                //
> +                // For reference, see __WW_CLASS_INITIALIZER() in
> +                // "include/linux/ww_mutex.h".
> +                mutex_key: unsafe { core::mem::zeroed() },

Does `..pin_init::zeroed()` work in this case?

Best,
Gary

> +            }),
> +        }
> +    }
> +
> +    /// Creates a [`Class`] from a raw pointer.
> +    ///
> +    /// This function is intended for interoperability with C code.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller must ensure that `ptr` points to the `inner` field of
> +    /// [`Class`] and that it remains valid for the lifetime `'a`.
> +    #[inline]
> +    pub const unsafe fn from_raw<'a>(ptr: *mut bindings::ww_class) -> &'a Self {
> +        // SAFETY: By the safety contract, `ptr` is valid to construct `Class`.
> +        unsafe { &*ptr.cast() }
> +    }
> +}
> +
> +// SAFETY: [`Class`] is immutable except for its atomic stamp. It's fine to share it across threads.
> +unsafe impl Sync for Class {}
> +// SAFETY: Doesn't hold anything thread-specific. It's safe to send to other threads.
> +unsafe impl Send for Class {}