Re: [PATCH v8 5/6] rust: ww_mutex: add Mutex, AcquireCtx and MutexGuard

[Alice Ryhl <aliceryhl@google.com>]

On Mon, Dec 01, 2025 at 01:28:54PM +0300, Onur Özkan wrote:
> Covers the entire low-level locking API (lock, try_lock,
> slow path, interruptible variants) and integration with
> kernel bindings.
> 
> Signed-off-by: Onur Özkan <work@onurozkan.dev>

> +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 {

Should also require that the class is valid for the duration of 'class.

> +/// 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>> {
> +    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);
> +            }

Hmm, this originates from the previous conversation:

https://lore.kernel.org/all/20251124184928.30b8bbaf@nimda/
>>> +    ///         // SAFETY: Both `lock_set` and `mutex1` uses the
>>> same class.
>>> +    ///         unsafe { lock_set.lock(&mutex1)? };
>>> +    ///
>>> +    ///         // SAFETY: Both `lock_set` and `mutex2` uses the
>>> same class.
>>> +    ///         unsafe { lock_set.lock(&mutex2)? };
>> 
>> I wonder if there's some way we can get rid of the safety contract
>> here and verify this at compile time, it would be a shame if every
>> single lock invocation needed to be unsafe.
>> 
> 
> Yeah :(. We could get rid of them easily by keeping the class that was
> passed to the constructor functions but that becomes a problem for the
> from_raw implementations.
> 
> I think the best solution would be to expose ww_class type from
> ww_acquire_ctx and ww_mutex unconditionally (right now it depends on
> DEBUG_WW_MUTEXES). That way we can just access the class and verify
> that the mutex and acquire_ctx classes match.
> 
> What do you think? I can submit a patch for the C-side implementation.
> It should be straightforward and shouldn't have any runtime impact.

I think there is a better solution. We can create a different type for
every single class, like how rust/kernel/sync/lock/global.rs creates a
different type for every single mutex. Then, you know that the classes
are the same since the class is part of the type.

Alice