Re: [PATCH 1/1] rust: sync: Add Lock::from_raw() for ZST data types

[Gary Guo <gary@garyguo.net>]

On Mon, 16 Sep 2024 18:05:46 -0400
Lyude Paul <lyude@redhat.com> 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
> 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<T, B>`, then given `T` is a ZST -
> `Lock<T, 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<T, 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>
> ---
>  rust/kernel/sync/lock.rs | 32 ++++++++++++++++++++++++++++++--
>  1 file changed, 30 insertions(+), 2 deletions(-)
> 
> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> index f6c34ca4d819f..f77cb178840b2 100644
> --- a/rust/kernel/sync/lock.rs
> +++ b/rust/kernel/sync/lock.rs
> @@ -6,8 +6,8 @@
>  //! spinlocks, raw spinlocks) to be provided with minimal effort.
>  
>  use super::LockClassKey;
> -use crate::{init::PinInit, pin_init, str::CStr, types::Opaque, types::ScopeGuard};
> -use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned};
> +use crate::{init::PinInit, pin_init, prelude::*, str::CStr, types::Opaque, types::ScopeGuard};
> +use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned, mem};
>  use macros::pin_data;
>  
>  pub mod mutex;
> @@ -81,6 +81,7 @@ 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.
> @@ -117,6 +118,33 @@ pub fn new(t: T, name: &'static CStr, key: &'static LockClassKey) -> impl PinIni
>              }),
>          })
>      }
> +
> +    /// Constructs a [`Lock`] from a raw pointer.
> +    ///
> +    /// This can be useful for interacting with a lock which was initialised outside of rust. This
> +    /// can only be used when `T` is a ZST type.
> +    ///
> +    /// # Safety
> +    ///
> +    /// - The caller promises that `ptr` points to a valid initialised instance of [`State`].
> +    /// - The caller promises that `T` is a type that it is allowed to create (e.g. `!` would not be
> +    ///   allowed)

I think "allowed to create" is quite vague. Is `IrqDisabled<'static>`
something that is classified as allowed to create? It is not an
uninhabited type, but it does have invariants associated with that.

Do you have a need for `T` to be arbitrary type, not just `()`? I would
prefer to see this being an impl on `Lock<(), B>` if you do not have a
specific need for arbitrary ZST types.

Best,
Gary

> +    ///
> +    /// [`State`]: Backend::State
> +    pub unsafe fn from_raw<'a>(ptr: *mut B::State) -> &'a Self {
> +        build_assert!(
> +            mem::size_of::<T>() == 0,
> +            "Lock::<T, B>::from_raw() can only be used if T is a ZST"
> +        );
> +
> +        // SAFETY:
> +        // * By the safety contract `ptr` must point to a valid initialised instance of `B::State`
> +        // * We just asserted that `T` is a ZST, making `state` the only non-ZST member of the
> +        //   struct
> +        // * Combined with `#[repr(C)]`, this guarantees `Self` has an equivalent data layout to
> +        //   `B::State`.
> +        unsafe { &*ptr.cast() }
> +    }
>  }
>  
>  impl<T: ?Sized, B: Backend> Lock<T, B> {