linux-kernel - Re: [PATCH V3] implement `ww_mutex` abstraction for the Rust tree

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Message-Id: <DAQL8YH3LDKW.341ZTFFLTTUA2@kernel.org>
Date: Thu, 19 Jun 2025 16:43:58 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: Onur Özkan <work@...rozkan.dev>,
 <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Cc: <peterz@...radead.org>, <mingo@...hat.com>, <will@...nel.org>,
 <boqun.feng@...il.com>, <longman@...hat.com>, <ojeda@...nel.org>,
 <alex.gaynor@...il.com>, <gary@...yguo.net>, <bjorn3_gh@...tonmail.com>,
 <a.hindborg@...nel.org>, <aliceryhl@...gle.com>, <tmgross@...ch.edu>,
 <dakr@...nel.org>, <thatslyude@...il.com>
Subject: Re: [PATCH V3] implement `ww_mutex` abstraction for the Rust tree

On Thu Jun 19, 2025 at 4:06 PM CEST, Onur Özkan wrote:
> From: onur-ozkan <work@...rozkan.dev>
>
> Adds Rust bindings for the kernel's `ww_mutex` infrastructure to enable
> deadlock-free acquisition of multiple related locks.
>
> The implementation abstracts `ww_mutex.h` header and wraps the existing
> C `ww_mutex` with three main types:
>     - `WwClass` for grouping related mutexes
>     - `WwAcquireCtx` for tracking lock acquisition context
>     - `WwMutex<T>` for the actual lock
>
> Some of the kernel's `ww_mutex` functions are implemented as `static inline`,
> so they are inaccessible from Rust as bindgen can't generate code on them.
> The `rust/helpers/ww_mutex.c` file provides C function wrappers around these inline
> implementations, so bindgen can see them and generate the corresponding Rust code.

I don't know the design of `struct ww_mutex`, but from the code below I
gathered that it has some special error return values that signify that
one should release other locks.

Did anyone think about making a more Rusty API that would allow one to
try to lock multiple mutexes at the same time (in a specified order) and
if it fails, it would do the resetting automatically?

---
Cheers,
Benno

> Link: https://rust-for-linux.zulipchat.com/#narrow/channel/291566-Library/topic/Writing.20up.20wrappers.20for.20ww_mutex.3F/with/524269974
> Suggested-by: thatslyude@...il.com
> Signed-off-by: Onur Özkan <work@...rozkan.dev>
> ---
>  rust/helpers/helpers.c            |   1 +
>  rust/helpers/ww_mutex.c           |  39 +++
>  rust/kernel/error.rs              |   1 +
>  rust/kernel/sync/lock.rs          |   1 +
>  rust/kernel/sync/lock/ww_mutex.rs | 556 ++++++++++++++++++++++++++++++
>  5 files changed, 598 insertions(+)
>  create mode 100644 rust/helpers/ww_mutex.c
>  create mode 100644 rust/kernel/sync/lock/ww_mutex.rs