Re: [PATCH 1/2] rust: introduce `InPlaceModule`

[Wedson Almeida Filho]

On Wed, 27 Mar 2024 at 05:13, Valentin Obst <kernel@xxxxxxxxxxxxxxx> wrote:
>
> > This allows modules to be initialised in-place in pinned memory, which
> > enables the usage of pinned types (e.g., mutexes, spinlocks, driver
> > registrations, etc.) in modules without any extra allocations.
> >
> > Drivers that don't need this may continue to implement `Module` without
> > any changes.
> >
> > Signed-off-by: Wedson Almeida Filho <walmeida@xxxxxxxxxxxxx>
> > ---
> >  rust/kernel/lib.rs    | 25 ++++++++++++++++++++++++-
> >  rust/macros/module.rs | 18 ++++++------------
> >  2 files changed, 30 insertions(+), 13 deletions(-)
> >
> > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> > index 5c641233e26d..64aee4fbc53b 100644
> > --- a/rust/kernel/lib.rs
> > +++ b/rust/kernel/lib.rs
> > @@ -62,7 +62,7 @@
> >  /// The top level entrypoint to implementing a kernel module.
> >  ///
> >  /// For any teardown or cleanup operations, your type may implement [`Drop`].
> > -pub trait Module: Sized + Sync {
> > +pub trait Module: Sized + Sync + Send {
>
> This does not compile with `CONFIG_AX88796B_RUST_PHY=y || m` (or the
> phylib abstractions' doctests) since the module `Registration` is not
> `Send`.

Thanks for the heads up. I thought I had enabled all rust code but
indeed I was missing this. I will fix it in v2.

> I remember Trevor raising the question whether we want to require modules
> to be `Send`. I am not aware of any examples of `!Send` modules but I guess
> it would be possible to write code that is only correct under the
> assumption that it is loaded/unloaded in the same context.

It might be possible in the future, but I don't believe it is now
because all rust modules support unloading. And there is no guarantee
that the thread unloading (and therefore calling module_exit) is the
same that loaded (and called module_init), so a module must be Send to
properly handle drop being called from a different thread.

Not requiring Send on the original Module trait was an oversight that
I don't want to repeat in InPlaceModule.

>
> @Trevor: Are you aware of any modules with that requirement?
>
> I have been using this patch for quite a while with my TCP CCAs now
> (without the `Send` bound) and did not experience any other issues; thus
> offering:
>         Tested-by: Valentin Obst <kernel@xxxxxxxxxxxxxxx>

Thanks!

>
>         - Best Valentin
>
> >      /// Called at module initialization time.
> >      ///
> >      /// Use this method to perform whatever setup or registration your module
> > @@ -72,6 +72,29 @@ pub trait Module: Sized + Sync {
> >      fn init(module: &'static ThisModule) -> error::Result<Self>;
> >  }
> >
> > +/// A module that is pinned and initialised in-place.
> > +pub trait InPlaceModule: Sync + Send {
> > +    /// Creates an initialiser for the module.
> > +    ///
> > +    /// It is called when the module is loaded.
> > +    fn init(module: &'static ThisModule) -> impl init::PinInit<Self, error::Error>;
> > +}
> > +
> > +impl<T: Module> InPlaceModule for T {
> > +    fn init(module: &'static ThisModule) -> impl init::PinInit<Self, error::Error> {
> > +        let initer = move |slot: *mut Self| {
> > +            let m = <Self as Module>::init(module)?;
> > +
> > +            // SAFETY: `slot` is valid for write per the contract with `pin_init_from_closure`.
> > +            unsafe { slot.write(m) };
> > +            Ok(())
> > +        };
> > +
> > +        // SAFETY: On success, `initer` always fully initialises an instance of `Self`.
> > +        unsafe { init::pin_init_from_closure(initer) }
> > +    }
> > +}
> > +
> >  /// Equivalent to `THIS_MODULE` in the C API.
> >  ///
> >  /// C header: [`include/linux/export.h`](srctree/include/linux/export.h)
> > diff --git a/rust/macros/module.rs b/rust/macros/module.rs
> > index 27979e582e4b..0b2bb4ec2fba 100644
> > --- a/rust/macros/module.rs
> > +++ b/rust/macros/module.rs
> > @@ -208,7 +208,7 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
> >              #[used]
> >              static __IS_RUST_MODULE: () = ();
> >
> > -            static mut __MOD: Option<{type_}> = None;
> > +            static mut __MOD: core::mem::MaybeUninit<{type_}> = core::mem::MaybeUninit::uninit();
> >
> >              // SAFETY: `__this_module` is constructed by the kernel at load time and will not be
> >              // freed until the module is unloaded.
> > @@ -275,23 +275,17 @@ pub(crate) fn module(ts: TokenStream) -> TokenStream {
> >              }}
> >
> >              fn __init() -> core::ffi::c_int {{
> > -                match <{type_} as kernel::Module>::init(&THIS_MODULE) {{
> > -                    Ok(m) => {{
> > -                        unsafe {{
> > -                            __MOD = Some(m);
> > -                        }}
> > -                        return 0;
> > -                    }}
> > -                    Err(e) => {{
> > -                        return e.to_errno();
> > -                    }}
> > +                let initer = <{type_} as kernel::InPlaceModule>::init(&THIS_MODULE);
> > +                match unsafe {{ initer.__pinned_init(__MOD.as_mut_ptr()) }} {{
> > +                    Ok(m) => 0,
> > +                    Err(e) => e.to_errno(),
> >                  }}
> >              }}
> >
> >              fn __exit() {{
> >                  unsafe {{
> >                      // Invokes `drop()` on `__MOD`, which should be used for cleanup.
> > -                    __MOD = None;
> > +                    __MOD.assume_init_drop();
> >                  }}
> >              }}
> >
> > --