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Date:   Wed, 31 May 2023 14:07:49 +0000
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Message-ID: <20230531140749.3708572-1-aliceryhl@google.com>
Subject: Re: [PATCH v1 6/7] rust: workqueue: add safe API to workqueue
From:   Alice Ryhl <aliceryhl@google.com>
To:     nmi@metaspace.dk
Cc:     alex.gaynor@gmail.com, aliceryhl@google.com,
        benno.lossin@proton.me, bjorn3_gh@protonmail.com,
        boqun.feng@gmail.com, gary@garyguo.net, jiangshanlai@gmail.com,
        linux-kernel@vger.kernel.org, ojeda@kernel.org,
        patches@lists.linux.dev, rust-for-linux@vger.kernel.org,
        tj@kernel.org, wedsonaf@gmail.com
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Andreas Hindborg <nmi@metaspace.dk> writes:
> Alice Ryhl <aliceryhl@google.com> writes:
>> +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> };
> 
> We have this functionality in the default impl of
> `HasWork<T>::raw_get_work() where Self: Sized`. I am uncertain about the
> `Sized` bound. If it is sound to do the offset calculation here where
> `T: ?Sized`, it should also be sound in the default implementation of
> `HasWork<T>`. Should we not be able to change the bound on
> `HasWork<T>::raw_get_work()` to `Self: ?Sized` and call into that from
> here?
> 
>         let work_ptr = unsafe { <T as HasWork<Self>>::raw_get_work(ptr as _) };
> 
> Same for Box.
> 
> BR Andreas

I looked into this, and it seems like we can remove `Self: Sized` bound
from `raw_get_work`, so I am able to simplify this code somewhat.
However, it cannot be removed from `container_of`.

Thanks,
Alice