linux-kernel - Re: [PATCH RFC v6 01/26] rust: clist: Add support to interface with C linked lists

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Message-ID: <237f94cc-dde6-4d91-b942-4854e62eeebb@joelfernandes.org>
Date: Wed, 21 Jan 2026 13:12:55 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: Zhi Wang <zhiw@...dia.com>, Joel Fernandes <joelagnelf@...dia.com>
Cc: linux-kernel@...r.kernel.org,
 Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
 Maxime Ripard <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>,
 David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
 Jonathan Corbet <corbet@....net>, Alex Deucher <alexander.deucher@....com>,
 Christian König <christian.koenig@....com>,
 Jani Nikula <jani.nikula@...ux.intel.com>,
 Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
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 <tursulin@...ulin.net>, Huang Rui <ray.huang@....com>,
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 <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>,
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 Alexandre Courbot <acourbot@...dia.com>, Andrea Righi <arighi@...dia.com>,
 Andy Ritger <aritger@...dia.com>, Alexey Ivanov <alexeyi@...dia.com>,
 Balbir Singh <balbirs@...dia.com>, Philipp Stanner <phasta@...nel.org>,
 Elle Rhumsaa <elle@...thered-steel.dev>,
 Daniel Almeida <daniel.almeida@...labora.com>,
 nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
 rust-for-linux@...r.kernel.org, linux-doc@...r.kernel.org,
 amd-gfx@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
 intel-xe@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
 Boqun Feng <boqun.feng@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH RFC v6 01/26] rust: clist: Add support to interface with C
 linked lists

On 1/21/2026 2:27 AM, Zhi Wang wrote:
> 
>> +/// Initialize a `list_head` object to point to itself.
>> +///
>> +/// # Safety
>> +///
>> +/// `list` must be a valid pointer to a `list_head` object.
>> +#[inline]
>> +pub unsafe fn init_list_head(list: *mut bindings::list_head) {
>> +    // SAFETY: Caller guarantees `list` is a valid pointer to a
>> `list_head`.
>> +    unsafe {
>> +        (*list).next = list;
>> +        (*list).prev = list;
>> +    }
>> +}
>> +
>
> Might be better to have a C helper? since INIT_LIST_HEAD() has WRITE_ONCE()
> for memory ordering. This one seems not equal to it.

WRITE_ONCE() is not really about CPU memory ordering though, it is about
compiler optimizations. On the C side, I think it is needed in case of
list_for_each_entry_rcu(), to avoid the case of invented stores or store fusing,
but here we are not doing RCU-based iteration.

Anyway, if we want to future proof that, I am Ok with adding the helper back
(which I actually initially had but feedback from past review was to just inline
it into rust).

But I am not sure if we have this issue for the Rust compiler, like we do for C.
Rust does not allow raw pointers to be concurrently read/written using plain
accesses, so should be already protected due to the borrow checker and compiler
itself right?

Adding some interested folks as well to CC for the topic of _ONCE, +Boqun +Paul.

--
Joel Fernandes