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

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Message-ID: <87ms2pgu7c.fsf@t14s.mail-host-address-is-not-set>
Date: Wed, 07 Jan 2026 19:21:11 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: fujita.tomonori@...il.com, aliceryhl@...gle.com, lyude@...hat.com,
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Subject: Re: [PATCH 4/5] rust: hrtimer: use READ_ONCE instead of read_volatile

"FUJITA Tomonori" <fujita.tomonori@...il.com> writes:

> On Wed, 07 Jan 2026 11:11:43 +0100
> Andreas Hindborg <a.hindborg@...nel.org> wrote:
>
>> FUJITA Tomonori <fujita.tomonori@...il.com> writes:
>>
>>> On Tue, 06 Jan 2026 13:37:34 +0100
>>> Andreas Hindborg <a.hindborg@...nel.org> wrote:
>>>
>>>> "FUJITA Tomonori" <fujita.tomonori@...il.com> writes:
>>>>
>>>>> On Thu, 01 Jan 2026 11:11:23 +0900 (JST)
>>>>> FUJITA Tomonori <fujita.tomonori@...il.com> wrote:
>>>>>
>>>>>> On Wed, 31 Dec 2025 12:22:28 +0000
>>>>>> Alice Ryhl <aliceryhl@...gle.com> wrote:
>>>>>>
>>>>>>> Using `READ_ONCE` is the correct way to read the `node.expires` field.
>>>>>>>
>>>>>>> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
>>>>>>> ---
>>>>>>>  rust/kernel/time/hrtimer.rs | 8 +++-----
>>>>>>>  1 file changed, 3 insertions(+), 5 deletions(-)
>>>>>>>
>>>>>>> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
>>>>>>> index 856d2d929a00892dc8eaec63cebdf547817953d3..e2b7a26f8aade972356c3eb5f6489bcda3e2e849 100644
>>>>>>> --- a/rust/kernel/time/hrtimer.rs
>>>>>>> +++ b/rust/kernel/time/hrtimer.rs
>>>>>>> @@ -239,11 +239,9 @@ pub fn expires(&self) -> HrTimerInstant<T>
>>>>>>>          // - Timers cannot have negative ktime_t values as their expiration time.
>>>>>>>          // - There's no actual locking here, a racy read is fine and expected
>>>>>>>          unsafe {
>>>>>>> -            Instant::from_ktime(
>>>>>>> -                // This `read_volatile` is intended to correspond to a READ_ONCE call.
>>>>>>> -                // FIXME(read_once): Replace with `read_once` when available on the Rust side.
>>>>>>> -                core::ptr::read_volatile(&raw const ((*c_timer_ptr).node.expires)),
>>>>>>> -            )
>>>>>>> +            Instant::from_ktime(kernel::sync::READ_ONCE(
>>>>>>> +                &raw const (*c_timer_ptr).node.expires,
>>>>>>> +            ))
>>>>>>>          }
>>>>>>
>>>>>> Do we actually need READ_ONCE() here? I'm not sure but would it be
>>>>>> better to call the C-side API?
>>>>>>
>>>>>> diff --git a/rust/helpers/time.c b/rust/helpers/time.c
>>>>>> index 67a36ccc3ec4..73162dea2a29 100644
>>>>>> --- a/rust/helpers/time.c
>>>>>> +++ b/rust/helpers/time.c
>>>>>> @@ -2,6 +2,7 @@
>>>>>>
>>>>>>  #include <linux/delay.h>
>>>>>>  #include <linux/ktime.h>
>>>>>> +#include <linux/hrtimer.h>
>>>>>>  #include <linux/timekeeping.h>
>>>>>>
>>>>>>  void rust_helper_fsleep(unsigned long usecs)
>>>>>> @@ -38,3 +39,8 @@ void rust_helper_udelay(unsigned long usec)
>>>>>>  {
>>>>>>  	udelay(usec);
>>>>>>  }
>>>>>> +
>>>>>> +__rust_helper ktime_t rust_helper_hrtimer_get_expires(const struct hrtimer *timer)
>>>>>> +{
>>>>>> +	return timer->node.expires;
>>>>>> +}
>>>>>
>>>>> 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.
>>>
>>> What does "racy read" mean here?
>>>
>>> The C side doesn't use WRITE_ONCE() or READ_ONCE for node.expires. How
>>> would using READ_ONCE() on the Rust side make a difference?
>>
>> Data races like this are UB in Rust. As far as I understand, using this
>> READ_ONCE implementation or a relaxed atomic read would make the read
>> well defined. I am not aware if this is only the case if all writes to
>> the location from C also use atomic operations or WRITE_ONCE. @Boqun?
>
> The C side updates node.expires without WRITE_ONCE()/atomics so a
> Rust-side READ_ONCE() can still observe a torn value; I think that
> this is still a data race / UB from Rust's perspective.
>
> And since expires is 64-bit, WRITE_ONCE() on 32-bit architectures does
> not inherently guarantee tear-free stores either.
>
> I think that the expires() method should follow the same safety
> requirements as raw_forward(): it should only be considered safe when
> holding exclusive access to hrtimer or within the context of the timer
> callback. Under those conditions, it would be fine to call C's
> hrtimer_get_expires().

We can make it safe, please see my comment here [1].

Best regards,
Andreas Hindborg

[1] https://lore.kernel.org/r/87v7hdh9m4.fsf@t14s.mail-host-address-is-not-set