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

Greg KH
2025-03-24T13:30:52Z

On Mon, Mar 24, 2025 at 12:59:27PM +0100, Danilo Krummrich wrote:
> 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
>

I can't "drop" anything here as that would be a mess. Maybe we just
consider a merge of the driver core and rust trees at this point in time
and fix things up and do a combined pull request to Linus so he doesn't
have to deal with the fixups?

thanks,

greg k-h