Re: [RFC PATCH] rust: workqueue: Add an example for try_spawn()

["Benno Lossin" <lossin@kernel.org>]

On Wed Jul 30, 2025 at 6:34 PM CEST, Boqun Feng wrote:
> `try_spawn()` could use an example to demonstrate the usage, and
> arguably it's the most simple usage of workqueue in case someone needs a
> deferred work, so add it.
>
> Cc: Joel Fernandes <joelagnelf@nvidia.com>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
> Miguel, Alice and Tejun, while I'm at it, should we also rename the
> function to `spawn()` because of the motivation mentioned here [1]?
>
> [1]: https://rust-for-linux.zulipchat.com/#narrow/channel/291566-Library/topic/.60new.60.20or.20.60try_new.60.3F/near/529533317
>
> Also I find the `{ <clone> || { } }` is really good if I only need to
> clone the Arc for passing to a callback closure, but I'm not sure how
> people feel about it, so criticism is welcome ;-)

I'm not so sure, see below :)

>  rust/kernel/workqueue.rs | 27 +++++++++++++++++++++++++++
>  1 file changed, 27 insertions(+)
>
> diff --git a/rust/kernel/workqueue.rs b/rust/kernel/workqueue.rs
> index b9343d5bc00f..59c1a5e14d12 100644
> --- a/rust/kernel/workqueue.rs
> +++ b/rust/kernel/workqueue.rs
> @@ -331,6 +331,33 @@ pub fn enqueue_delayed<W, const ID: u64>(&self, w: W, delay: Jiffies) -> W::Enqu
>      /// Tries to spawn the given function or closure as a work item.
>      ///
>      /// This method can fail because it allocates memory to store the work item.
> +    ///
> +    /// # Examples
> +    ///
> +    /// ```
> +    /// use kernel::{alloc::flags, sync::{Arc, Completion, new_spinlock}, workqueue};
> +    ///
> +    /// let work_done = Arc::pin_init(Completion::new(), flags::GFP_KERNEL)?;
> +    /// let data = Arc::pin_init(new_spinlock!(0), flags::GFP_KERNEL)?;
> +    ///
> +    /// workqueue::system().try_spawn(
> +    ///     flags::GFP_KERNEL,
> +    ///     {
> +    ///         let work_done = work_done.clone();
> +    ///         let data = data.clone();
> +    ///         move || {
> +    ///             *data.lock() = 42;
> +    ///             work_done.complete_all();
> +    ///         }
> +    ///     }
> +    /// )?;

Not doing your pattern and instead adding a `2` postfix we get:

    let work_done2 = work_done.clone();
    let data2 = data.clone();

    workqueue::system().try_spawn(flags::GFP_KERNEL, move || {
        *data2.lock() = 42;
        work_done2.complete_all();
    })?;

There are some discussions of introducing some better syntax for (cheap)
cloning, so maybe we can use that in the future.

---
Cheers,
Benno

> +    ///
> +    /// work_done.wait_for_completion();
> +    ///
> +    /// // `work_done` being completed implies the observation of the write of `data` in the work.
> +    /// assert_eq!(*data.lock(), 42);
> +    /// # Ok::<(), Error>(())
> +    /// ```
>      pub fn try_spawn<T: 'static + Send + FnOnce()>(
>          &self,
>          flags: Flags,