Date:   Tue, 11 Apr 2023 00:01:54 -0300
From:   Wedson Almeida Filho <wedsonaf@...il.com>
To:     Martin Rodriguez Reboredo <yakoyoku@...il.com>
Cc:     rust-for-linux@...r.kernel.org, Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        linux-kernel@...r.kernel.org,
        Wedson Almeida Filho <walmeida@...rosoft.com>
Subject: Re: [PATCH v3 02/13] rust: sync: introduce `Lock` and `Guard`

On Sun, 9 Apr 2023 at 13:47, Martin Rodriguez Reboredo
<yakoyoku@...il.com> wrote:
>
> On 4/8/23 04:53, Wedson Almeida Filho wrote:
> > From: Wedson Almeida Filho <walmeida@...rosoft.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.
> >
> > Suggested-by: Gary Guo <gary@...yguo.net>
> > Signed-off-by: Wedson Almeida Filho <walmeida@...rosoft.com>
> > ---
> > v1 -> v2: No changes
> > v2 -> v3: Use new Opaque::ffi_init from Benno's series
> >
> >  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..3fa4eefde740
> > --- /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`.
> > +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.
> > +    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.
> > +    #[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
> > +/// specified as the generic parameter `T`.
>
> I think this meant that it is specified by `B` and wraps `T`.

Yes, I meant `B` here. Will fix.

> > +#[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.
> > +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
> > +        // 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
> > +/// 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,
> > +        }
> > +    }
> > +}
>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@...il.com>

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