linux-kernel - Re: [PATCH v6 2/3] rust: sync: Introduce lock::Backend::Context

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Message-ID: <ZvnoOaOQudoVyBjF@boqun-archlinux>
Date: Sun, 29 Sep 2024 16:52:25 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Lyude Paul <lyude@...hat.com>
Cc: rust-for-linux@...r.kernel.org, Danilo Krummrich <dakr@...hat.com>,
	airlied@...hat.com, Ingo Molnar <mingo@...hat.com>,
	Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Benno Lossin <benno.lossin@...ton.me>,
	Daniel Almeida <daniel.almeida@...labora.com>,
	Gary Guo <gary@...yguo.net>, Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	Martin Rodriguez Reboredo <yakoyoku@...il.com>,
	Valentin Obst <kernel@...entinobst.de>
Subject: Re: [PATCH v6 2/3] rust: sync: Introduce lock::Backend::Context

On Mon, Sep 16, 2024 at 05:28:05PM -0400, Lyude Paul wrote:
> Now that we've introduced an `IrqDisabled` token for marking contexts in
> which IRQs are disabled, we need a way to be able to pass it to locks that
> require that IRQs are disabled. In order to continue using the
> `lock::Backend` type instead of inventing our own thing, we accomplish this
> by adding the associated Context type, along with a `lock_with()` function
> that can accept a Context when acquiring a lock. To allow current users of
> context-less locks to keep using the normal `lock()` method, we take an
> example from Wedson Almeida Filho's work and add a `where T<'a>: Default`
> bound to `lock()` so that it can only be called on lock types where the
> context is simply a placeholder value, then re-implement it through the new
> `lock_with()` function.
> 

Given that Backend::Context and "lock() where Context: Default" are
basically Benno's idea, I think it's fair to have a Suggested-by tag
here.

> Signed-off-by: Lyude Paul <lyude@...hat.com>
> Reviewed-by: Benno Lossin <benno.lossin@...ton.me>
> Reviewed-by: Daniel Almeida <daniel.almeida@...labora.com>
> Reviewed-by: Gary Guo <gary@...yguo.net>
> 

Reviewed-by: Boqun Feng <boqun.feng@...il.com>

Regards,
Boqun

> ---
> 
> V3:
> * Use explicit lifetimes in lock_with() to ensure self and _context have
>   the same lifetime (Benno)
> * Use () for locks that don't need a Context instead of PhantomData (Benno)
> V4:
> * Fix typo (Dirk)
> 
> ---
>  rust/kernel/sync/lock.rs          | 17 +++++++++++++++--
>  rust/kernel/sync/lock/mutex.rs    |  1 +
>  rust/kernel/sync/lock/spinlock.rs |  1 +
>  3 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> index d6e9bab114b87..cc39a3dc3f20b 100644
> --- a/rust/kernel/sync/lock.rs
> +++ b/rust/kernel/sync/lock.rs
> @@ -43,6 +43,9 @@ pub unsafe trait Backend {
>      /// [`unlock`]: Backend::unlock
>      type GuardState;
>  
> +    /// The context which must be provided to acquire the lock.
> +    type Context<'a>;
> +
>      /// Initialises the lock.
>      ///
>      /// # Safety
> @@ -125,14 +128,24 @@ pub fn new(t: T, name: &'static CStr, key: &'static LockClassKey) -> impl PinIni
>  }
>  
>  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> {
> +    /// 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> {
>          // 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) }
>      }
> +
> +    /// Acquires the lock and gives the caller access to the data protected by it.
> +    #[inline]
> +    pub fn lock<'a>(&'a self) -> Guard<'a, T, B>
> +    where
> +        B::Context<'a>: Default,
> +    {
> +        self.lock_with(B::Context::default())
> +    }
>  }
>  
>  /// A lock guard.
> diff --git a/rust/kernel/sync/lock/mutex.rs b/rust/kernel/sync/lock/mutex.rs
> index 30632070ee670..7c2c239944931 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 ea5c5bc1ce12e..97d85a5576615 100644
> --- a/rust/kernel/sync/lock/spinlock.rs
> +++ b/rust/kernel/sync/lock/spinlock.rs
> @@ -92,6 +92,7 @@ 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,
> -- 
> 2.46.0
>