linux-kernel - Re: [PATCH 4/5] rust: hrtimer: use READ_ONCE instead of read

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Message-ID: <f974d17e-0eeb-44a6-8a4a-a1bdab5af97c@nvidia.com>
Date: Tue, 6 Jan 2026 18:59:26 -0800
From: John Hubbard <jhubbard@...dia.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Alice Ryhl <aliceryhl@...gle.com>, Gary Guo <gary@...yguo.net>,
 Andreas Hindborg <a.hindborg@...nel.org>,
 FUJITA Tomonori <fujita.tomonori@...il.com>, lyude@...hat.com,
 will@...nel.org, peterz@...radead.org, richard.henderson@...aro.org,
 mattst88@...il.com, linmag7@...il.com, catalin.marinas@....com,
 ojeda@...nel.org, bjorn3_gh@...tonmail.com, lossin@...nel.org,
 tmgross@...ch.edu, dakr@...nel.org, mark.rutland@....com,
 frederic@...nel.org, tglx@...utronix.de, anna-maria@...utronix.de,
 jstultz@...gle.com, sboyd@...nel.org, viro@...iv.linux.org.uk,
 brauner@...nel.org, jack@...e.cz, linux-kernel@...r.kernel.org,
 linux-alpha@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 rust-for-linux@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 4/5] rust: hrtimer: use READ_ONCE instead of read_volatile

On 1/6/26 5:08 PM, Boqun Feng wrote:
> On Tue, Jan 06, 2026 at 04:47:35PM -0800, John Hubbard wrote:
>> On 1/6/26 10:43 AM, Alice Ryhl wrote:
>>> On Tue, Jan 06, 2026 at 03:23:00PM +0000, Gary Guo wrote:
>>>> On Tue, 06 Jan 2026 13:37:34 +0100
>>>> Andreas Hindborg <a.hindborg@...nel.org> wrote:
>>>>> "FUJITA Tomonori" <fujita.tomonori@...il.com> writes:
...
>>>>> This is a potentially racy read. As far as I recall, we determined that
>>>>> using read_once is the proper way to handle the situation.
>>>>>
>>>>> I do not think it makes a difference that the read is done by C code.
>>>>
>>>> If that's the case I think the C code should be fixed by inserting the
>>>> READ_ONCE?
>>>
>>> I maintain my position that if this is what you recommend C code does,
>>> it's confusing to not make the same recommendation for Rust abstractions
>>> to the same thing.
>>>
>>> After all, nothing is stopping you from calling atomic_read() in C too.
>>>
>>
>> Hi Alice and everyone!
>>
>> I'm having trouble fully understanding the latest reply, so maybe what
>> I'm saying is actually what you just said.
>>
>> Anyway, we should use READ_ONCE in both the C and Rust code. Relying
>> on the compiler for that is no longer OK. We shouldn't be shy about
>> fixing the C side (not that I think you have been, so far!).
>>
> 
> Agreed on most of it, except that we should be more explicit in Rust,
> by using atomic_load[1] instead of READ_ONCE().
> 
> [1]: https://lore.kernel.org/rust-for-linux/aV0FxCRzXFrNLZik@tardis-2.local/
> 

I see. That does put things in a much clearer state, yes.

thanks,
-- 
John Hubbard