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

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Message-Id: <86E0C8EE-393D-4C6A-9C28-BB036A1FFAD6@collabora.com>
Date: Wed, 3 Dec 2025 14:23:14 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Onur Özkan <work@...rozkan.dev>,
 rust-for-linux@...r.kernel.org,
 lossin@...nel.org,
 lyude@...hat.com,
 ojeda@...nel.org,
 alex.gaynor@...il.com,
 boqun.feng@...il.com,
 gary@...yguo.net,
 a.hindborg@...nel.org,
 tmgross@...ch.edu,
 dakr@...nel.org,
 peterz@...radead.org,
 mingo@...hat.com,
 will@...nel.org,
 longman@...hat.com,
 felipe_life@...e.com,
 daniel@...lak.dev,
 thomas.hellstrom@...ux.intel.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 5/6] rust: ww_mutex: add Mutex, AcquireCtx and
 MutexGuard



> On 3 Dec 2025, at 10:26, Alice Ryhl <aliceryhl@...gle.com> wrote:
> 
> 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@...rozkan.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.

I don’t think this would work with the from_raw() functions. What class
would you assign then? I think this is precisely what sparked the current
solution.

> 
> Alice