Re: [PATCH v1 6/7] rust: workqueue: add safe API to workqueue

[Alice Ryhl <aliceryhl@google.com>]

On 5/18/23 21:17, Martin Rodriguez Reboredo wrote:
> On 5/17/23 17:31, Alice Ryhl wrote:
>> +unsafe impl<T> WorkItem for Arc<T>
>> +where
>> +    T: ArcWorkItem + HasWork<Self> + ?Sized,
>> +{
>> +    type EnqueueOutput = Result<(), Self>;
>> +
>> +    unsafe fn __enqueue<F>(self, queue_work_on: F) -> Self::EnqueueOutput
>> +    where
>> +        F: FnOnce(*mut bindings::work_struct) -> bool,
>> +    {
>> +        let ptr = Arc::into_raw(self);
>> +
>> +        // Using `get_work_offset` here for object-safety.
>> +        //
>> +        // SAFETY: The pointer is valid since we just got it from `into_raw`.
>> +        let off = unsafe { (&*ptr).get_work_offset() };
>> +
>> +        // SAFETY: The `HasWork` impl promises that this offset gives us a field of type
>> +        // `Work<Self>` in the same allocation.
>> +        let work_ptr = unsafe { (ptr as *const u8).add(off) as *const Work<Self> };
>> +        // SAFETY: The pointer is not dangling.
>> +        let work_ptr = unsafe { Work::raw_get(work_ptr) };
>> +
>> +        match (queue_work_on)(work_ptr) {
> 
> Match for boolean is not a good pattern in my eyes, if-else should be
> used instead.

I think this is a question of style. For a comparison:

match (queue_work_on)(work_ptr) {
    true => Ok(()),
    // SAFETY: The work queue has not taken ownership of the pointer.
    false => Err(unsafe { Arc::from_raw(ptr) }),
}

vs

if (queue_work_on)(work_ptr) {
    Ok(())
} else {
    // SAFETY: The work queue has not taken ownership of the pointer.
    Err(unsafe { Arc::from_raw(ptr) }),
}

I'm happy to change it if others disagree, but when the branches
evaluate to a short expression like they do here, I quite like the first
version.

> Also aren't the parens around the closure unnecessary?

Hmm, parenthesises are often required around closures, but it's possible
that it is only required for stuff like `self.closure(args)` to
disambiguate between a `closure` field (of pointer type) and a `closure`
method. I can check and remove them if they are not necessary.