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

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Message-ID: <20251215121001.5a0cc42c@nimda>
Date: Mon, 15 Dec 2025 12:10:01 +0300
From: Onur Özkan <work@...rozkan.dev>
To: Alice Ryhl <aliceryhl@...gle.com>
Cc: Daniel Almeida <daniel.almeida@...labora.com>,
 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 Thu, 4 Dec 2025 09:07:18 +0000
Alice Ryhl <aliceryhl@...gle.com> wrote:

> On Wed, Dec 03, 2025 at 02:23:14PM -0300, Daniel Almeida wrote:
> > 
> > 
> > > 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:
> > >> 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.
> 
> There can be a way to create a type for a C-defined class, and
> from_raw() can require that you don't use the same Rust type for
> different C classes.
> 

Do you think this is a better alternative? IMO it doesn't seem worth
it for what it's doing. Current approach adds less complexity and is
easier to maintain. It's not just helping from_raw functions, the class
validation is being much simpler without having to deal with storing
class references or creating new types.

I am holding off the next version because we don't have a clear
consensus on this.

- Onur

> Alice