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

[Danilo Krummrich <dakr@kernel.org>]

Hi Stephen,

On Mon, Mar 24, 2025 at 10:06:29PM +1100, Stephen Rothwell wrote:
> Hi all,
> 
> On Mon, 24 Mar 2025 21:57:02 +1100 Stephen Rothwell <sfr@canb.auug.org.au> wrote:
> >
> > After merging the rust tree, today's linux-next build (x86_64
> > allmodconfig) failed like this:
> > 
> > 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
> >    |
> >    = help: within `DmaSampleDriver`, the trait `Send` is not implemented for `*mut MyStruct`, which is required by `DmaSampleDriver: Send`
> > note: required because it appears within the type `CoherentAllocation<MyStruct>`
> >   --> rust/kernel/dma.rs:132:12  
> > note: required because it appears within the type `DmaSampleDriver`
> >   --> samples/rust/rust_dma.rs:9:8  
> >    |
> > 9  | struct DmaSampleDriver {
> >    |        ^^^^^^^^^^^^^^^
> > note: required by a bound in `kernel::pci::Driver`
> >   --> rust/kernel/pci.rs:225:1  
> > 
> > error: aborting due to 1 previous error
> > 
> > For more information about this error, try `rustc --explain E0277`.
> > 
> > I have no idea what caused this - it built in next-20250321, but that
> > no longer builds, so I have reset to the version of the rust tree in
> > next-20250320 (commit 4a47eec07be6).
> 
> Actually, the driver-core tree gained these commits over the weekend:
> 
>   51d0de7596a4 ("rust: platform: require Send for Driver trait implementers")
>   935e1d90bf6f ("rust: pci: require Send for Driver trait implementers")
>   455943aa187f ("rust: platform: impl Send + Sync for platform::Device")
>   e2942bb4e629 ("rust: pci: impl Send + Sync for pci::Device")
> 
> A heads up would have been nice ... and maybe even a test merge and
> build against -next (given how late we are in the cycle).

Commit 935e1d90bf6f ("rust: pci: require Send for Driver trait implementers")
from the driver-core tree fixes a missing concurrency requirement, which commit
9901addae63b ("samples: rust: add Rust dma test sample driver") from the Rust
tree did not yet consider.

Technically, it did what it is supposed to do -- catch a concurrency issue at
compile time. However, since I was involved into both sides, I could have
thought of this, but unfortunately in this case it was too subtle for me to
spot -- sorry.

There are two options, 1. simply drop the commit [1] that introduces the
affected sample DMA code, or 2. apply the fix below to [2]. My preference would
be (2).

--

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

--

[1] 9901addae63b ("samples: rust: add Rust dma test sample driver")
[2] ad2907b4e308 ("rust: add dma coherent allocator abstraction")