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

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Message-ID: <aTGNlO0C86pJMFdj@google.com>
Date: Thu, 4 Dec 2025 13:33:08 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Daniel Almeida <daniel.almeida@...labora.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 Thu, Dec 04, 2025 at 07:26:25AM -0600, Daniel Almeida wrote:
> 
> 
> > On 4 Dec 2025, at 03:07, 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
> 
> That’s the problem, if we don’t have patch 2, we don’t know the class.
> 
> What you’re suggesting seems unimplementable to me at first. Otherwise, can
> you expand some more?

For each class defined by C code, you invoke a macro:

	ww_class_from_c_code(MY_C_CLASS, bindings::my_c_class);

Then when you call from_raw(), you call

	Mutex::<T, MY_C_CLASS>::from_raw(ptr_to_mutex)

There is no need for a check, because from_raw() is unsafe.

Alice