Message-ID: <87ldi7f4o1.fsf@t14s.mail-host-address-is-not-set>
Date: Fri, 09 Jan 2026 11:42:38 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: FUJITA Tomonori <fujita.tomonori@...il.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 19:21:11 +0100
> Andreas Hindborg <a.hindborg@...nel.org> wrote:
>
>> "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
>
> I agree. My point was that expire() can be safe only under the same
> constraints as forward()/forward_now() so the API should require
> Pin<&mut Self> and expose it on HrTimerCallbackContext.

Do you want to send a patch?


Best regards,
Andreas Hindborg

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