[PATCH 4/5] rust: hrtimer: use READ_ONCE instead of read_volatile

[FUJITA Tomonori]

On Wed, 07 Jan 2026 11:11:43 +0100
Andreas Hindborg <a.hindborg at kernel.org> wrote:

> FUJITA Tomonori <fujita.tomonori at gmail.com> writes:
> 
>> On Tue, 06 Jan 2026 13:37:34 +0100
>> Andreas Hindborg <a.hindborg at kernel.org> wrote:
>>
>>> "FUJITA Tomonori" <fujita.tomonori at gmail.com> writes:
>>> 
>>>> On Thu, 01 Jan 2026 11:11:23 +0900 (JST)
>>>> FUJITA Tomonori <fujita.tomonori at gmail.com> wrote:
>>>>
>>>>> On Wed, 31 Dec 2025 12:22:28 +0000
>>>>> Alice Ryhl <aliceryhl at google.com> wrote:
>>>>>
>>>>>> Using `READ_ONCE` is the correct way to read the `node.expires` field.
>>>>>>
>>>>>> Signed-off-by: Alice Ryhl <aliceryhl at google.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().