Subject: Re: linux-next: build failure after merge of the rust tree

From: Danilo Krummrich <[email-masked]>
Date: Mon, 24 Mar 2025 16:27:28 +0100

On Mon, Mar 24, 2025 at 04:18:40PM +0100, Miguel Ojeda wrote:
> On Mon, Mar 24, 2025 at 2:43 PM Danilo Krummrich <[email-masked]> wrote:
> >
> > It's the DMA commit that has a bug, that was revealed by the fix in the
> > driver-core tree. So, the patch to drop is in the rust tree (not sure if Miguel
> > changes history at this point though).
> 
> Just to double-check, the diff you show below is the combined one,
> right? i.e. it is the one that Stephen already had the previous week +
> the fix you posted above (`Send` `impl`), right?

Correct, it is the full conflict resolution of the current rust and driver-core
tree.

> 
> If so, I think it is OK, and we could put the new `Send` impl on top
> of `rust-next` -- given the trees on their own are OK until they
> arrive to Linus, I am not sure if it counts as a fix.
> 
> i.e. something like the attached patch (crediting Danilo and Stephen).

Thanks, the attached patch looks perfectly fine to me to add on top.

One small nit: The "Link:" tag should rather be "Closes:".

> Cheers,
> Miguel

> From 6a152af23cb49a3bcbb8c4457a612ffa27d54693 Mon Sep 17 00:00:00 2001
> From: Danilo Krummrich <[email-masked]>
> Date: Mon, 24 Mar 2025 16:01:00 +0100
> Subject: [PATCH] rust: dma: add `Send` implementation for `CoherentAllocation`
> 
> Stephen found a future build failure in linux-next [1]:
> 
>     error[E0277]: `*mut MyStruct` cannot be sent between threads safely
>       --> samples/rust/rust_dma.rs:47:22
>        |
>     47 | impl pci::Driver for DmaSampleDriver {
>        |                      ^^^^^^^^^^^^^^^ `*mut MyStruct` cannot be sent between threads safely
> 
> It is caused by the interaction between commit 935e1d90bf6f ("rust: pci:
> require Send for Driver trait implementers") from the driver-core tree,
> which fixes a missing concurrency requirement, and commit 9901addae63b
> ("samples: rust: add Rust dma test sample driver") which adds a sample
> that does not satisfy that requirement.
> 
> Add a `Send` implementation to `CoherentAllocation`, which allows the
> sample (and other future users) to satisfy it.
> 
> Reported-by: Stephen Rothwell <[email-masked]>
> Link: https://lore.kernel.org/linux-next/[email-masked]/
> Signed-off-by: Danilo Krummrich <[email-masked]>
> Signed-off-by: Miguel Ojeda <[email-masked]>
> ---
>  rust/kernel/dma.rs | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/rust/kernel/dma.rs b/rust/kernel/dma.rs
> index 9d00f9c49f47..18de693c4924 100644
> --- a/rust/kernel/dma.rs
> +++ b/rust/kernel/dma.rs
> @@ -301,6 +301,10 @@ fn drop(&mut self) {
>      }
>  }
>  
> +// SAFETY: It is safe to send a `CoherentAllocation` to another thread if `T`
> +// can be send to another thread.
> +unsafe impl<T: AsBytes + FromBytes + Send> Send for CoherentAllocation<T> {}
> +
>  /// Reads a field of an item from an allocated region of structs.
>  ///
>  /// # Examples
> -- 
> 2.49.0
>