linux-kernel - Re: [PATCH v5 0/3] rust: add `ww

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Message-Id: <76D4D052-79B6-4D3F-AAA1-164FF7A41284@collabora.com>
Date: Thu, 14 Aug 2025 09:38:38 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Onur Özkan <work@...rozkan.dev>
Cc: Benno Lossin <lossin@...nel.org>,
 Lyude Paul <lyude@...hat.com>,
 linux-kernel@...r.kernel.org,
 rust-for-linux@...r.kernel.org,
 ojeda@...nel.org,
 alex.gaynor@...il.com,
 boqun.feng@...il.com,
 gary@...yguo.net,
 a.hindborg@...nel.org,
 aliceryhl@...gle.com,
 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,
 bjorn3_gh@...tonmail.com
Subject: Re: [PATCH v5 0/3] rust: add `ww_mutex` support

Hi Onur,

> On 14 Aug 2025, at 08:13, Onur Özkan <work@...rozkan.dev> wrote:
> 
> Hi all,
> 
> I have been brainstorming on the auto-unlocking (on dynamic number of
> mutexes) idea we have been discussing for some time.
> 
> There is a challange with how we handle lock guards and my current
> thought is to remove direct data dereferencing from guards. Instead,
> data access would only be possible through a fallible method (e.g.,
> `try_get`). If the guard is no longer valid, this method would fail to
> not allow data-accessing after auto-unlock.
> 
> In practice, it would work like this:
> 
> let a_guard = ctx.lock(mutex_a)?;
> let b_guard = ctx.lock(mutex_b)?;
> 
> // Suppose user tries to lock `mutex_c` without aborting the
> // entire function (for some reason). This means that even on
> // failure, `a_guard` and `b_guard` will still be accessible.
> if let Ok(c_guard) = ctx.lock(mutex_c) {
>     // ...some logic
> }
> 
> let a_data = a_guard.try_get()?;
> let b_data = b_guard.try_get()?;

Can you add more code here? How is this going to look like with the two
closures we’ve been discussing?

> 
> If user wants to access the data protected by `a_guard` or `b_guard`,
> they must call `try_get()`. This will only succeed if the guard is
> still valid (i.e., it hasn't been automatically unlocked by a failed
> `lock(mutex_c)` call due to `EDEADLK`). This way, data access after an
> auto-unlock will be handled safely.
> 
> Any thoughts/suggestions?
> 
> Regards,
> Onur
> 

— Daniel