Re: [PATCH RFC 06/11] rust: sync: Replace static LockClassKey refs with a pointer wrapper

[Martin Rodriguez Reboredo <yakoyoku@gmail.com>]

On 7/14/23 06:13, Asahi Lina wrote:
> We want to be able to handle dynamic lock class creation and using
> pointers to things that aren't a real lock_class_key as lock classes.
> Doing this by casting around Rust references is difficult without
> accidentally invoking UB.
> 
> Instead, switch LockClassKey to being a raw pointer wrapper around a
> lock_class_key, which means there is no UB possible on the Rust side
> just by creating and consuming these objects. The C code also should
> never actually dereference lock classes, only use their address
> (possibly with an offset).
> 
> We still provide a dummy ZST version of this wrapper, to be used when
> lockdep is disabled.
> 
> Signed-off-by: Asahi Lina <lina@asahilina.net>
> ---
> [...]
> diff --git a/rust/kernel/sync/lockdep.rs b/rust/kernel/sync/lockdep.rs
> index cb68b18dc0ad..d8328f4275fb 100644
> --- a/rust/kernel/sync/lockdep.rs
> +++ b/rust/kernel/sync/lockdep.rs
> @@ -9,19 +9,36 @@
>   
>   /// Represents a lockdep class. It's a wrapper around C's `lock_class_key`.
>   #[repr(transparent)]
> -pub struct LockClassKey(Opaque<bindings::lock_class_key>);
> +pub struct StaticLockClassKey(Opaque<bindings::lock_class_key>);
>   
> -impl LockClassKey {
> +impl StaticLockClassKey {
>       /// Creates a new lock class key.
>       pub const fn new() -> Self {
>           Self(Opaque::uninit())
>       }
>   
> +    /// Returns the lock class key reference for this static lock class.
> +    pub const fn key(&self) -> LockClassKey {
> +        LockClassKey(self.0.get())

`Opaque::get` is not a `const fn` so this will not compile.

> +    }
> +}
> +
> +// SAFETY: `bindings::lock_class_key` just represents an opaque memory location, and is never
> +// actually dereferenced.
> +unsafe impl Sync for StaticLockClassKey {}
> +
> +/// A reference to a lock class key. This is a raw pointer to a lock_class_key,
> +/// which is required to have a static lifetime.
> +#[derive(Copy, Clone)]
> +pub struct LockClassKey(*mut bindings::lock_class_key);
> +
> +impl LockClassKey {
>       pub(crate) fn as_ptr(&self) -> *mut bindings::lock_class_key {

I think this can be made into a `const fn`. What do you all think?

> -        self.0.get()
> +        self.0
>       }
>   }
>   
> [...]