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

[FUJITA Tomonori]

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);
+}

> diff --git a/rust/kernel/time/hrtimer.rs b/rust/kernel/time/hrtimer.rs
> index 856d2d929a00..61e656a65216 100644
> --- a/rust/kernel/time/hrtimer.rs
> +++ b/rust/kernel/time/hrtimer.rs
> @@ -237,14 +237,7 @@ pub fn expires(&self) -> HrTimerInstant<T>
>  
>          // SAFETY:
>          // - 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)),
> -            )
> -        }
> +        unsafe { Instant::from_ktime(bindings::hrtimer_get_expires(c_timer_ptr)) }
>      }
>  }
>  
>