Re: [PATCH v4 02/13] rust: sync: introduce `Lock` and `Guard`

[Benno Lossin <benno.lossin@proton.me>]

On 11.04.23 07:45, Wedson Almeida Filho wrote:
> From: Wedson Almeida Filho <walmeida@microsoft.com>
>
> They are generic Rust implementations of a lock and a lock guard that
> contain code that is common to all locks. Different backends will be
> introduced in subsequent commits.
>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> Suggested-by: Gary Guo <gary@garyguo.net>
> Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
> ---
> v1 -> v2: No changes
> v2 -> v3: Use new Opaque::ffi_init from Benno's series
> v3 -> v4: Fixed name of parameter in Lock comment
>
>   rust/kernel/sync.rs      |   2 +-
>   rust/kernel/sync/lock.rs | 162 +++++++++++++++++++++++++++++++++++++++
>   2 files changed, 163 insertions(+), 1 deletion(-)
>   create mode 100644 rust/kernel/sync/lock.rs
>
> diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
> index 541d235ffbeb..81b0998eaa18 100644
> --- a/rust/kernel/sync.rs
> +++ b/rust/kernel/sync.rs
> @@ -8,6 +8,7 @@
>   use crate::types::Opaque;
>
>   mod arc;
> +pub mod lock;
>
>   pub use arc::{Arc, ArcBorrow, UniqueArc};
>
> @@ -25,7 +26,6 @@ impl LockClassKey {
>           Self(Opaque::uninit())
>       }
>
> -    #[allow(dead_code)]
>       pub(crate) fn as_ptr(&self) -> *mut bindings::lock_class_key {
>           self.0.get()
>       }
> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> new file mode 100644
> index 000000000000..1a8ecccf4f24
> --- /dev/null
> +++ b/rust/kernel/sync/lock.rs
> @@ -0,0 +1,162 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Generic kernel lock and guard.
> +//!
> +//! It contains a generic Rust lock and guard that allow for different backends (e.g., mutexes,
> +//! spinlocks, raw spinlocks) to be provided with minimal effort.
> +
> +use super::LockClassKey;
> +use crate::{bindings, init::PinInit, pin_init, str::CStr, types::Opaque};
> +use core::{cell::UnsafeCell, marker::PhantomData, marker::PhantomPinned};
> +use macros::pin_data;
> +
> +/// The "backend" of a lock.
> +///
> +/// It is the actual implementation of the lock, without the need to repeat patterns used in all
> +/// locks.
> +///
> +/// # Safety
> +///
> +/// - Implementers must ensure that only one thread/CPU may access the protected data once the lock
> +/// is owned, that is, between calls to `lock` and `unlock`.

I am a bit confused which data is protected. The `Backend` should not be
aware of any data it protects. To me the safety requirement is:
- Up to one thread may be in the critical section at any one time.
- When `lock` returns, the current thread has entered the critical
   section.
- A thread leaves the critical section by calling `unlock`.

> +pub unsafe trait Backend {
> +    /// The state required by the lock.
> +    type State;
> +
> +    /// The state required to be kept between lock and unlock.
> +    type GuardState;
> +
> +    /// Initialises the lock.
> +    ///
> +    /// # Safety
> +    ///
> +    /// `ptr` must be valid for write for the duration of the call, while `name` and `key` must
> +    /// remain valid for read indefinitely.

- Only call this function once on uninitialized memory.

Do we require some sort of drop guarantee like pin? This function does not
state anything about `State` having to be pinned, but `Lock` is `!Unpin`
(and also pins the state). If other people decide to use `Backend`
somewhere, these things should be clear.

> +    unsafe fn init(
> +        ptr: *mut Self::State,
> +        name: *const core::ffi::c_char,
> +        key: *mut bindings::lock_class_key,
> +    );
> +
> +    /// Acquires the lock, making the caller its owner.
> +    ///
> +    /// # Safety
> +    ///
> +    /// Callers must ensure that [`Backend::init`] has been previously called.

Is it safe to call this function twice? Does `unlock` have to be
called, or is one allowed to drop the value before?

> +    #[must_use]
> +    unsafe fn lock(ptr: *mut Self::State) -> Self::GuardState;
> +
> +    /// Releases the lock, giving up its ownership.
> +    ///
> +    /// # Safety
> +    ///
> +    /// It must only be called by the current owner of the lock.
> +    unsafe fn unlock(ptr: *mut Self::State, guard_state: &Self::GuardState);
> +}
> +
> +/// A mutual exclusion primitive.
> +///
> +/// Exposes one of the kernel locking primitives. Which one is exposed depends on the lock banckend

Typo: 'banckend' -> '[`Backend`]' (I would prefer a link here)

> +/// specified as the generic parameter `B`.
> +#[pin_data]
> +pub struct Lock<T: ?Sized, B: Backend> {
> +    /// The kernel lock object.
> +    #[pin]
> +    state: Opaque<B::State>,
> +
> +    /// Some locks are known to be self-referential (e.g., mutexes), while others are architecture
> +    /// or config defined (e.g., spinlocks). So we conservatively require them to be pinned in case
> +    /// some architecture uses self-references now or in the future.
> +    #[pin]
> +    _pin: PhantomPinned,
> +
> +    /// The data protected by the lock.
> +    data: UnsafeCell<T>,
> +}
> +
> +// SAFETY: `Lock` can be transferred across thread boundaries iff the data it protects can.

Are there backends where `Backend::State: !Send`? If so we should add
this as a bound, if not, there should be a comment here, since it is
unconditionally implemented. Same for `Sync`

> +unsafe impl<T: ?Sized + Send, B: Backend> Send for Lock<T, B> {}
> +
> +// SAFETY: `Lock` serialises the interior mutability it provides, so it is `Sync` as long as the
> +// data it protects is `Send`.
> +unsafe impl<T: ?Sized + Send, B: Backend> Sync for Lock<T, B> {}
> +
> +impl<T, B: Backend> Lock<T, B> {
> +    /// Constructs a new lock initialiser.
> +    #[allow(clippy::new_ret_no_self)]
> +    pub fn new(t: T, name: &'static CStr, key: &'static LockClassKey) -> impl PinInit<Self> {
> +        pin_init!(Self {
> +            data: UnsafeCell::new(t),
> +            _pin: PhantomPinned,
> +            // SAFETY: `slot` is valid while the closure is called and both `name` and `key` have
> +            // static lifetimes so they live indefinitely.
> +            state <- Opaque::ffi_init(|slot| unsafe {
> +                B::init(slot, name.as_char_ptr(), key.as_ptr())
> +            }),
> +        })
> +    }
> +}
> +
> +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> {
> +        // SAFETY: The constructor of the type calls `init`, so the existence of the object proves

I would make 'the type' concrete here: 'The constructor of `Self` ...'. And
then '... so the existence of `self` ...'.

> +        // that `init` was called.
> +        let state = unsafe { B::lock(self.state.get()) };
> +        // SAFETY: The lock was just acquired.
> +        unsafe { Guard::new(self, state) }
> +    }
> +}
> +
> +/// A lock guard.
> +///
> +/// Allows mutual exclusion primitives that implement the `Backend` trait to automatically unlock

Make `Backend` a link.

> +/// when a guard goes out of scope. It also provides a safe and convenient way to access the data
> +/// protected by the lock.
> +#[must_use = "the lock unlocks immediately when the guard is unused"]
> +pub struct Guard<'a, T: ?Sized, B: Backend> {
> +    pub(crate) lock: &'a Lock<T, B>,
> +    pub(crate) state: B::GuardState,
> +    _not_send: PhantomData<*mut ()>,
> +}
> +
> +// SAFETY: `Guard` is sync when the data protected by the lock is also sync.
> +unsafe impl<T: Sync + ?Sized, B: Backend> Sync for Guard<'_, T, B> {}
> +
> +impl<T: ?Sized, B: Backend> core::ops::Deref for Guard<'_, T, B> {
> +    type Target = T;
> +
> +    fn deref(&self) -> &Self::Target {
> +        // SAFETY: The caller owns the lock, so it is safe to deref the protected data.
> +        unsafe { &*self.lock.data.get() }
> +    }
> +}
> +
> +impl<T: ?Sized, B: Backend> core::ops::DerefMut for Guard<'_, T, B> {
> +    fn deref_mut(&mut self) -> &mut Self::Target {
> +        // SAFETY: The caller owns the lock, so it is safe to deref the protected data.
> +        unsafe { &mut *self.lock.data.get() }
> +    }
> +}
> +
> +impl<T: ?Sized, B: Backend> Drop for Guard<'_, T, B> {
> +    fn drop(&mut self) {
> +        // SAFETY: The caller owns the lock, so it is safe to unlock it.
> +        unsafe { B::unlock(self.lock.state.get(), &self.state) };
> +    }
> +}
> +
> +impl<'a, T: ?Sized, B: Backend> Guard<'a, T, B> {
> +    /// Constructs a new immutable lock guard.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The caller must ensure that it owns the lock.
> +    pub(crate) unsafe fn new(lock: &'a Lock<T, B>, state: B::GuardState) -> Self {
> +        Self {
> +            lock,
> +            state,
> +            _not_send: PhantomData,
> +        }
> +    }
> +}
> --
> 2.34.1
>