linux-kernel - Re: [PATCH] rust/revocable: add try

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Message-ID: <Z9L-Z4Aw64Hi5Lj8@pollux>
Date: Thu, 13 Mar 2025 16:48:55 +0100
From: Danilo Krummrich <dakr@...nel.org>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Alexandre Courbot <acourbot@...dia.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rust/revocable: add try_with() convenience method

On Thu, Mar 13, 2025 at 03:38:55PM +0000, Benno Lossin wrote:
> On Thu Mar 13, 2025 at 4:08 PM CET, Alexandre Courbot wrote:
> > On Thu Mar 13, 2025 at 11:19 PM JST, Benno Lossin wrote:
> >> Would it make sense to not use `Result` here and continue with `Option`?
> >
> > We would have to return an Option<Result<R>> in this case. The current
> > code folds the closure's Result into the one of the guard's acquisition
> > for convenience.
> >
> > Actually, I don't think I have ever used try_access() a single time
> > without converting its returned Option into a Result. Wouldn't it make
> > sense to do the opposite, i.e. make try_access() return Err(ENXIO) when
> > the guard cannot be acquired and document this behavior?
> 
> Sure, if you're always doing
> 
>     let guard = rev.try_access().ok_or(ENXIO)?;
> 
> Then it makes sense from my view, maybe Danilo has some other argument
> for why `Option` is better.

Most of the time I think we indeed want to derive an Err() if try_access()
fails, but not with a specific error code. The error code depends on the context
of where the revocable is used (e.g. for I/O mappings), but it also depends on
the driver semantics.