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

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

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.
>
>> ---
>>  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.