linux-kernel - Re: [PATCH v8 02/14] rust: hrtimer: introduce hrtimer support

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Message-ID: <CAJ-ks9kQccoa7znFNzWAgi6_G0TKvLUARWPZ_Dbed1C-d4Lr+Q@mail.gmail.com>
Date: Fri, 21 Feb 2025 09:46:08 -0500
From: Tamir Duberstein <tamird@...il.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Andreas Hindborg <a.hindborg@...nel.org>, Miguel Ojeda <ojeda@...nel.org>, 
	Anna-Maria Behnsen <anna-maria@...utronix.de>, Frederic Weisbecker <frederic@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>, Danilo Krummrich <dakr@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Alice Ryhl <aliceryhl@...gle.com>, 
	Trevor Gross <tmgross@...ch.edu>, Lyude Paul <lyude@...hat.com>, Guangbo Cui <2407018371@...com>, 
	Dirk Behme <dirk.behme@...il.com>, Daniel Almeida <daniel.almeida@...labora.com>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 02/14] rust: hrtimer: introduce hrtimer support

On Fri, Feb 21, 2025 at 9:40 AM Boqun Feng <boqun.feng@...il.com> wrote:
>
> Hmm... if you mean:
>
> trait HasHrTimer {
>     unsafe fn start(&self, expires: Ktime) {
>         ...
>     }
> }
>
> Then it'll be problematic because the pointer derived from `&self`
> doesn't have write provenance, therefore in a timer callback, the
> pointer cannot be used for write, which means for example you cannot
> convert the pointer back into a `Pin<Box<HasTimer>>`.
>
> To answer Tamir's question, pointers are heavily used here because we
> need to preserve the provenance.

Wouldn't the natural implication be that &mut self is needed? Maybe
you can help me understand why pointers can express a contract that
references can't?