Re: [PATCH v3 1/2] rust: sync: Add Lock::from_raw() for Lock<(), B>

[Boqun Feng <boqun.feng@gmail.com>]

Hi Lyude,

On Thu, Oct 31, 2024 at 07:08:44PM -0400, Lyude Paul wrote:
> A few of the APIs I've been writing bindings for (KMS in particular) rely
> on the user manually acquiring specific locks before calling certain
> functions. At the moment though, the only way of acquiring these locks in

Could you reword this in a more abstract/object way, like: KMS (probably
a link to you RFC patchset) has a few bindings that require manually
acquire specific locks before calling certain functions.

> bindings is to simply call the C locking functions directly - since said
> locks are not acquired on the rust side of things.
> 
> However - if we add `#[repr(C)]` to `Lock<(), B>`, then given `()` is a ZST
> - `Lock<(), B>` becomes equivalent in data layout to its inner `B::State`
> type. Since locks in C don't have data explicitly associated with them
> anyway, we can take advantage of this to add a `Lock::from_raw()` function
> that can translate a raw pointer to `B::State` into its proper `Lock<(),
> B>` equivalent. This lets us simply acquire a reference to the lock in
> question and work with it like it was initialized on the rust side of
> things, allowing us to use less unsafe code to implement bindings with lock
> requirements.
> 
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> 
> ---
> 
> V2:
> * Don't implement this for all ZST types, just implement it for ()
> V3:
> * Get rid of some unused imports
> * Make sure that we run rustfmt
> 
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> ---
>  rust/kernel/sync/lock.rs | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
> 
> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> index 90cc5416529bd..c7b0c6351f793 100644
> --- a/rust/kernel/sync/lock.rs
> +++ b/rust/kernel/sync/lock.rs
> @@ -88,6 +88,7 @@ unsafe fn relock(ptr: *mut Self::State, guard_state: &mut Self::GuardState) {
>  ///
>  /// Exposes one of the kernel locking primitives. Which one is exposed depends on the lock
>  /// [`Backend`] specified as the generic parameter `B`.
> +#[repr(C)]
>  #[pin_data]
>  pub struct Lock<T: ?Sized, B: Backend> {
>      /// The kernel lock object.
> @@ -126,6 +127,27 @@ pub fn new(t: T, name: &'static CStr, key: &'static LockClassKey) -> impl PinIni
>      }
>  }
>  
> +impl<B: Backend> Lock<(), B> {
> +    /// Constructs a [`Lock`] from a raw pointer.
> +    ///
> +    /// This can be useful for interacting with a lock which was initialised outside of rust.

s/rust/Rust

> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller promises that `ptr` points to a valid initialised instance of [`State`].

Please add "during the whole lifetime of `'a`" or something similar.

> +    ///
> +    /// [`State`]: Backend::State
> +    pub unsafe fn from_raw<'a>(ptr: *mut B::State) -> &'a Self {
> +        // SAFETY:
> +        // * By the safety contract `ptr` must point to a valid initialised instance of `B::State`
> +        // * Since the lock data type is `()` which is a ZST, `state` is the only non-ZST member of
> +        //   the struct
> +        // * Combined with `#[repr(C)]`, this guarantees `Self` has an equivalent data layout to
> +        //   `B::State`.

Please use '-' instead of '*' for listing to stay consistent with other
part of the file.

Regards,
Boqun

> +        unsafe { &*ptr.cast() }
> +    }
> +}
> +
>  impl<T: ?Sized, B: Backend> Lock<T, B> {
>      /// Acquires the lock and gives the caller access to the data protected by it.
>      pub fn lock(&self) -> Guard<'_, T, B> {
> -- 
> 2.47.0
>