Re: [PATCH v5 02/15] rust: sync: completion: add wait_for_completion_timeout()

["Alexandre Courbot" <acourbot@nvidia.com>]

On Fri Oct 2, 2026 at 8:13 PM JST, John Hubbard wrote:
> On 10/2/26 12:55 PM, Alexandre Courbot wrote:
>> On Fri Oct 2, 2026 at 6:50 PM JST, Gary Guo wrote:
>>> On Wed Sep 30, 2026 at 4:41 AM BST, John Hubbard wrote:
>>>> From: Joel Fernandes <joelagnelf@nvidia.com>
>>>>
>>>> A driver that runs an interrupt self-test during probe waits for the
>>>> handler to fire. wait_for_completion() has no timeout, so a broken
>>>> interrupt path stalls probe indefinitely. Add a timeout variant of
>>>> wait_for_completion().
>>>>
>>>> Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
>>>> Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
>>>> [jhubbard: return the remaining jiffies]
>>>> Signed-off-by: John Hubbard <jhubbard@nvidia.com>
>>>
>>> This email should have RfL list CC'ed.
>
> Hi Gary! Apologies, my send-email scripts had drifted into a
> nova-centric direction. Will fix.
>
>
>>>
>>>> ---
>>>>  rust/kernel/sync/completion.rs | 23 ++++++++++++++++++++++-
>>>>  1 file changed, 22 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/rust/kernel/sync/completion.rs b/rust/kernel/sync/completion.rs
>>>> index 35ff049ff078..7e8b3c1c880e 100644
>>>> --- a/rust/kernel/sync/completion.rs
>>>> +++ b/rust/kernel/sync/completion.rs
>>>> @@ -6,7 +6,12 @@
>>>>  //!
>>>>  //! C header: [`include/linux/completion.h`](srctree/include/linux/completion.h)
>>>>  
>>>> -use crate::{bindings, prelude::*, types::Opaque};
>>>> +use crate::{
>>>> +    bindings,
>>>> +    prelude::*,
>>>> +    time::Jiffies,
>>>> +    types::Opaque, //
>>>> +};
>>>>  
>>>>  /// Synchronization primitive to signal when a certain task has been completed.
>>>>  ///
>>>> @@ -111,4 +116,20 @@ pub fn wait_for_completion(&self) {
>>>>          // SAFETY: `self.as_raw()` is a pointer to a valid `struct completion`.
>>>>          unsafe { bindings::wait_for_completion(self.as_raw()) };
>>>>      }
>>>> +
>>>> +    /// Wait for completion of a task, with a timeout.
>>>> +    ///
>>>> +    /// This method waits for the completion of a task, or until `timeout` elapses. It is not
>>>> +    /// interruptible. Returns the number of jiffies left when the task completed, or [`None`] if
>>>> +    /// `timeout` elapsed first.
>>>> +    ///
>>>> +    /// See also [`Completion::complete_all`].
>>>> +    #[inline]
>>>> +    pub fn wait_for_completion_timeout(&self, timeout: Jiffies) -> Option<Jiffies> {
>>>
>>> NACK. Please use the typed `Delta<Jiffy>`, not this old type alias for c_ulong.
>>> I asked Miguel to take that late last cycle because I know there'll be new users
>>> that want to use Jiffies this cycle!
>>>
>>> This would avoid you having to do clamping on the max side (but you still need
>>> to clamp the value to be at least 0). There'll be a helper method for this, but
>>> it isn't available yet.
>>> https://lore.kernel.org/rust-for-linux/20261002050428.2746277-3-tomo@flapping.org/
>> 
>> I will drop this patch and replace the code using this new method (which
>> is just an optional probe-time self-test) by a polling loop on an
>> atomic. That way we can wait for the helper method to be available and
>> revisit this with the proper type.
>
> OK, whatever works best. Just in case, I did stage a fixed-up version
> of the patch locally (in case I need to send a v6), as per Gary's
> recommended Delta approach. It looks like this:

It's your series so your call, but since the elegant solution will
involve using [1] which is still under review, I would suggest waiting
for it to introduce `wait_for_completion_timeout`, moving forward with
the rest of the series, and revisit using completions once the base code
is ready to limit churn.

Otherwise we are at risk of missing the -rc6 cutoff for this series and
that means an even busier next cycle. The polling code is also simpler
overall than using completions, and for a probe-time selftest polling is
really not a problem. Nova is not losing anything by not using
completions for this.

[1] https://lore.kernel.org/rust-for-linux/20261002050428.2746277-3-tomo@flapping.org/