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

[FUJITA Tomonori]

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?