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

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Message-ID: <4f3f87ad-62f0-4557-8371-123a2306f573@nvidia.com>
Date: Tue, 6 Jan 2026 16:47:35 -0800
From: John Hubbard <jhubbard@...dia.com>
To: Alice Ryhl <aliceryhl@...gle.com>, Gary Guo <gary@...yguo.net>
Cc: Andreas Hindborg <a.hindborg@...nel.org>,
 FUJITA Tomonori <fujita.tomonori@...il.com>, lyude@...hat.com,
 boqun.feng@...il.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 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:
>>>>
>>>> Sorry, of course this should be:
>>>>
>>>> +__rust_helper ktime_t rust_helper_hrtimer_get_expires(const struct hrtimer *timer)
>>>> +{
>>>> +	return hrtimer_get_expires(timer);
>>>> +}
>>>>  
>>>
>>> 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!).

thanks,
-- 
John Hubbard