Re: [POC 5/6] rust: sync: Introduce lock::Backend::Context

[Lyude Paul <lyude@redhat.com>]

On Thu, 2024-10-17 at 22:51 -0700, Boqun Feng wrote:
> From: Lyude Paul <lyude@redhat.com>
> 
> Now that we've introduced an `InterruptDisabled` token for marking
> contexts in which IRQs are disabled, we can have a way to avoid
> `SpinLockIrq` disabling interrupts if the interrupts have already been
> disabled. Basically, a `SpinLockIrq` should work like a `SpinLock` if
> interrupts are disabled. So a function:
> 
> 	(&'a SpinLockIrq, &'a InterruptDisabled) -> Guard<'a, .., SpinLockBackend>
> 
> makes senses. Note that due to `Guard` and `InterruptDisabled` has the
> same lifetime, interrupts cannot be enabled whiel the Guard exists.
> 
> Add a `lock_with()` interface for `Lock`, and an associate type of
> `Backend` to describe the context.
> 
> [Boqun: Change the interface a lot, now `SpinLockIrq` can use the
> `lock()` function, but it always disable the interrupts, reuse the
> `lock_with()` method to provide a way for locking if interrupts are
> already disabled. `lock_with()` implementation will be added later.]
> 
> Co-developed-by: Lyude Paul <lyude@redhat.com>
> Signed-off-by: Lyude Paul <lyude@redhat.com>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
>  rust/kernel/sync/lock.rs          | 12 +++++++++++-
>  rust/kernel/sync/lock/mutex.rs    |  1 +
>  rust/kernel/sync/lock/spinlock.rs |  3 +++
>  3 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> index f6c34ca4d819..49b53433201c 100644
> --- a/rust/kernel/sync/lock.rs
> +++ b/rust/kernel/sync/lock.rs
> @@ -38,6 +38,9 @@ pub unsafe trait Backend {
>      /// [`unlock`]: Backend::unlock
>      type GuardState;
>  
> +    /// The context which can be provided to acquire the lock with a different backend.
> +    type Context<'a>;
> +
>      /// Initialises the lock.
>      ///
>      /// # Safety
> @@ -120,8 +123,15 @@ pub fn new(t: T, name: &'static CStr, key: &'static LockClassKey) -> impl PinIni
>  }
>  
>  impl<T: ?Sized, B: Backend> Lock<T, B> {
> +    /// Acquires the lock with the given context and gives the caller access to the data protected
> +    /// by it.
> +    pub fn lock_with<'a>(&'a self, _context: B::Context<'a>) -> Guard<'a, T, B> {
> +        todo!()
> +    }
> +
>      /// Acquires the lock and gives the caller access to the data protected by it.
> -    pub fn lock(&self) -> Guard<'_, T, B> {
> +    #[inline]
> +    pub fn lock<'a>(&'a self) -> Guard<'a, 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()) };
> diff --git a/rust/kernel/sync/lock/mutex.rs b/rust/kernel/sync/lock/mutex.rs
> index 30632070ee67..7c2c23994493 100644
> --- a/rust/kernel/sync/lock/mutex.rs
> +++ b/rust/kernel/sync/lock/mutex.rs
> @@ -93,6 +93,7 @@ macro_rules! new_mutex {
>  unsafe impl super::Backend for MutexBackend {
>      type State = bindings::mutex;
>      type GuardState = ();
> +    type Context<'a> = ();
>  
>      unsafe fn init(
>          ptr: *mut Self::State,
> diff --git a/rust/kernel/sync/lock/spinlock.rs b/rust/kernel/sync/lock/spinlock.rs
> index 884d4d1cbf23..8f9e1b27e474 100644
> --- a/rust/kernel/sync/lock/spinlock.rs
> +++ b/rust/kernel/sync/lock/spinlock.rs
> @@ -3,6 +3,7 @@
>  //! A kernel spinlock.
>  //!
>  //! This module allows Rust code to use the kernel's `spinlock_t`.
> +use crate::interrupt::InterruptDisabled;
>  
>  /// Creates a [`SpinLock`] initialiser with the given name and a newly-created lock class.
>  ///
> @@ -92,6 +93,7 @@ macro_rules! new_spinlock {
>  unsafe impl super::Backend for SpinLockBackend {
>      type State = bindings::spinlock_t;
>      type GuardState = ();
> +    type Context<'a> = ();
>  
>      unsafe fn init(
>          ptr: *mut Self::State,
> @@ -183,6 +185,7 @@ macro_rules! new_spinlock_irq {
>  unsafe impl super::Backend for SpinLockIrqBackend {
>      type State = bindings::spinlock_t;
>      type GuardState = ();
> +    type Context<'a> = &'a InterruptDisabled;

Does this actually need to be a reference here? I thought we wanted to use
just plain token types with lifetimes instead of references

>  
>      unsafe fn init(
>          ptr: *mut Self::State,

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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