linux-kernel - Re: [PATCH v9 3/5] rust: debugfs: Support `PinInit` backing for `File`s.

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Message-ID: <CAGSQo03meaA2fmoHqxdcVSXrKdE7yTTK0JfWH28mHZ=0kWoTzA@mail.gmail.com>
Date: Tue, 19 Aug 2025 07:33:31 -0700
From: Matthew Maurer <mmaurer@...gle.com>
To: Dirk Behme <dirk.behme@...bosch.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>, 
	Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org>, 
	Sami Tolvanen <samitolvanen@...gle.com>, Timur Tabi <ttabi@...dia.com>, 
	Benno Lossin <lossin@...nel.org>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org
Subject: Re: [PATCH v9 3/5] rust: debugfs: Support `PinInit` backing for `File`s.

On Mon, Aug 18, 2025 at 10:51 PM Dirk Behme <dirk.behme@...bosch.com> wrote:
>
> Hi Matthew,
>
> On 09/07/2025 21:09, Matthew Maurer wrote:
> > This allows `File`s to own their data, allowing DebugFS files to be
> > managed in sync with the data that backs them. Because DebugFS files are
> > intended to actually own data and provide access, `File`s still maintain
> > the same lifecycle for provided data when `CONFIG_DEBUG_FS` is disabled.
> >
> > Signed-off-by: Matthew Maurer <mmaurer@...gle.com>
> > ---
> >  rust/kernel/debugfs.rs | 149 ++++++++++++++++++++++++++++++++++++++-----------
> >  1 file changed, 117 insertions(+), 32 deletions(-)
> >
> > diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
> > index e5b6497d1deb67671d22ffd90cd5baa855bb9257..a1a84dd309216f455ae8fe3d3c0fd00f957f82a9 100644
> > --- a/rust/kernel/debugfs.rs
> > +++ b/rust/kernel/debugfs.rs
> > @@ -5,12 +5,13 @@
> >  //!
> >  //! C header: [`include/linux/debugfs.h`](srctree/include/linux/debugfs.h)
> >
> > -#[cfg(CONFIG_DEBUG_FS)]
> > -use crate::prelude::GFP_KERNEL;
> > +use crate::prelude::*;
> >  use crate::str::CStr;
> >  #[cfg(CONFIG_DEBUG_FS)]
> >  use crate::sync::Arc;
> >  use core::fmt::Display;
> > +use core::marker::PhantomPinned;
> > +use core::ops::Deref;
> >
> >  #[cfg(CONFIG_DEBUG_FS)]
> >  mod display_file;
> > @@ -63,40 +64,78 @@ fn create(_name: &CStr, _parent: Option<&Dir>) -> Self {
> >      }
> >
> >      #[cfg(CONFIG_DEBUG_FS)]
> > -    fn create_file<T: Display + Sized + Sync>(&self, name: &CStr, data: &'static T) -> File {
> > -        let Some(parent) = &self.0 else {
> > -            return File {
> > +    /// Creates a DebugFS file which will own the data produced by the initializer provided in
> > +    /// `data`.
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// The provided vtable must be appropriate for implementing a seq_file if provided
> > +    /// with a private data pointer which provides shared access to a `T`.
> > +    unsafe fn create_file<'a, T: Sync, E, TI: PinInit<T, E>>(
> > +        &self,
> > +        name: &'a CStr,
> > +        data: TI,
> > +        vtable: &'static bindings::file_operations,
> > +    ) -> impl PinInit<File<T>, E> + use<'_, 'a, T, E, TI> {
>
> Rebasing my test code from an older version of this series to this v9
> (this is the most recent one?) here in rust/kernel/debugfs.rs and in
> samples/rust/rust_debugfs.rs I get errors for each place where
> 'use<...>' is used:
>
> error[E0658]: precise captures on `impl Trait` are experimental
>   --> rust/kernel/debugfs.rs:81:37
>    |
> 81 |     ) -> impl PinInit<File<T>, E> + use<'_, 'a, T, E, TI> {
>    |                                     ^^^
>    |
>    = note: see issue #123432
> <https://github.com/rust-lang/rust/issues/123432> for more information
>    = help: add `#![feature(precise_capturing)]` to the crate attributes
> to enable
>    = note: this compiler was built on 2024-09-04; consider upgrading it
> if it is out of date
>
> rustc is
>
> rustc 1.81.0 (eeb90cda1 2024-09-04)
>
The easy way to resolve this is to use `rustc` 1.82.0 or newer (you're
only one revision behind!), but that's past the MSRV for the kernel at
the moment of 1.79.0. I intend to send up a new patch today, so I
suppose I'll need to put that behind a flag similar to
`RUSTC_HAS_DERIVE_COERCE_POINTEE`.
>
> I tried to work around this by adding
>
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -36,6 +36,9 @@
>  //
>  // To be determined.
>  #![feature(used_with_arg)]
> +
> +#![feature(precise_capturing)]
> +
>
> This seems to help for rust/kernel/debugfs.rs but not for
> samples/rust/rust_debugfs.rs.
Did you add `#![feature(precise_capturing)]` to
`samples/rust/rust_debugfs.rs` as well? It is its own crate, so it has
its own set of features.
>
>
> Any hint?
>
> Best regards
>
> Dirk
>