linux-kernel - Re: [PATCH 1/3] rust: completion: implement initial abstraction

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Message-Id: <DAKHYVHNSO30.3SICBU2MAW9UL@kernel.org>
Date: Thu, 12 Jun 2025 12:53:45 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Danilo Krummrich" <dakr@...nel.org>
Cc: <gregkh@...uxfoundation.org>, <rafael@...nel.org>, <ojeda@...nel.org>,
 <alex.gaynor@...il.com>, <boqun.feng@...il.com>, <gary@...yguo.net>,
 <bjorn3_gh@...tonmail.com>, <benno.lossin@...ton.me>,
 <a.hindborg@...nel.org>, <aliceryhl@...gle.com>, <tmgross@...ch.edu>,
 <chrisi.schrefl@...il.com>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, "Ingo Molnar" <mingo@...hat.com>, "Peter
 Zijlstra" <peterz@...radead.org>, "Juri Lelli" <juri.lelli@...hat.com>,
 "Vincent Guittot" <vincent.guittot@...aro.org>, "Dietmar Eggemann"
 <dietmar.eggemann@....com>, "Steven Rostedt" <rostedt@...dmis.org>, "Ben
 Segall" <bsegall@...gle.com>, "Mel Gorman" <mgorman@...e.de>, "Valentin
 Schneider" <vschneid@...hat.com>
Subject: Re: [PATCH 1/3] rust: completion: implement initial abstraction

On Thu Jun 12, 2025 at 12:35 PM CEST, Danilo Krummrich wrote:
> On Thu, Jun 12, 2025 at 10:15:55AM +0200, Benno Lossin wrote:
>> On Tue Jun 3, 2025 at 10:48 PM CEST, Danilo Krummrich wrote:
>> > +    /// Signal all tasks waiting on this completion.
>> > +    ///
>> > +    /// This method wakes up all tasks waiting on this completion; after this operation the
>> > +    /// completion is permanently done.
>> > +    pub fn complete_all(&self) {
>> > +        // SAFETY: `self.as_raw()` is a pointer to a valid `struct completion`.
>> > +        unsafe { bindings::complete_all(self.as_raw()) };
>> > +    }
>> > +
>> > +    /// Wait for completion of a task.
>> > +    ///
>> > +    /// This method waits for the completion of a task; it is not interruptible and there is no
>> > +    /// timeout.
>> 
>> Another thing that we should document is weather this function returns
>> immediately when `complete_all` was already called in the past.
>
> The details are all documented in [1], which is also linked in the module
> documentation of this file.
>
> [1] https://docs.kernel.org/scheduler/completion.html

I dislike that we don't have the docs right there on the function.
Following that link, there is also a lot of other stuff there that don't
apply to Rust (eg initializing completions, and the
wait_for_completion*() variants).

After a bit of reading, I found the part that I was looking for (by
searching for `complete_all`...):

    A thread that wants to signal that the conditions for continuation have
    been achieved calls `complete()` to signal exactly one of the waiters
    that it can continue:
    
    ```c
    void complete(struct completion *done)
    ```
    
    ... or calls `complete_all()` to signal all current and future waiters:
    
    ```c
    void complete_all(struct completion *done)
    ```

Let's just put this information on the `complete_all` function.

---
Cheers,
Benno