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

[FUJITA Tomonori]

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;
+}
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)) }
     }
 }