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

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Message-ID: <20250814141302.1eabda12@nimda.home>
Date: Thu, 14 Aug 2025 14:13:02 +0300
From: Onur Özkan <work@...rozkan.dev>
To: "Benno Lossin" <lossin@...nel.org>
Cc: "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 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 If the guard is no longer valid, this method would fail to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()?;

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