linux-kernel - Re: Rust kernel policy

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Message-ID: <20250219161543.GI19203@1wt.eu>
Date: Wed, 19 Feb 2025 17:15:43 +0100
From: Willy Tarreau <w@....eu>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: James Bottomley <James.Bottomley@...senPartnership.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Dan Carpenter <dan.carpenter@...aro.org>,
        Christoph Hellwig <hch@...radead.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        rust-for-linux <rust-for-linux@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>, David Airlie <airlied@...il.com>,
        linux-kernel@...r.kernel.org, ksummit@...ts.linux.dev
Subject: Re: Rust kernel policy

On Wed, Feb 19, 2025 at 06:07:23PM +0200, Laurent Pinchart wrote:

> > Regardless I do understand how these cleanups can help in a number of
> > case, at least to avoid some code duplication.
> 
> They're particularly useful to "destroy" local variables that don't need
> to be returned. This allows implementing scope guards, to facilitate
> lock handling for instance. Once a mutex guard is instantiated, the
> mutex is locked, and it will be guaranteed to be unlocked in every
> return path.

Yeah absolutely. However I remember having faced code in the past where
developers had abused this "unlock on return" concept resulting in locks
lazily being kept way too long after an operation. I don't think this
will happen in the kernel thanks to reviews, but typically all the stuff
that's done after a locked retrieval was done normally is down outside
of the lock, while here for the sake of not dealing with unlocks, quite
a few lines were still covered by the lock for no purpose. Anyway
there's no perfect solution.

Ideally when a compiler is smart enough to say "I would have cleaned
up here", it could be cool to just have a warning so that the developer
decides where to perform it. The problem is that it'd quickly becomes
a mess since the compiler cannot guess that you've done your own cleanup
before (without yet other anotations), which precisely is the point of
doing it unconditionally when leaving scope.

Willy