Re: [PATCH v2 2/3] rust: sync: Introduce lock::Backend::Context

[Lyude Paul <lyude@redhat.com>]

On Thu, 2024-08-01 at 10:11 +0000, Benno Lossin wrote:
> On 01.08.24 00:35, 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.
> > 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> > ---
> >  rust/kernel/sync/lock.rs          | 17 +++++++++++++++--
> >  rust/kernel/sync/lock/mutex.rs    |  3 +++
> >  rust/kernel/sync/lock/spinlock.rs |  1 +
> >  3 files changed, 19 insertions(+), 2 deletions(-)
> > 
> > diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> > index f6c34ca4d819f..f3ff50c4272cf 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 much be provided to acquire the lock.
> > +    type Context<'a>;
> > +
> >      /// Initialises the lock.
> >      ///
> >      /// # Safety
> > @@ -120,14 +123,24 @@ 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(&self, _context: B::Context<'_>) -> Guard<'_, T, B> {
> 
> Here we need to be careful, without lifetime elision, this signature is:
> 
>     pub fn lock_with<'a, 'b>(&'a self, _context: B::Context<'b>) -> Guard<'a, T, B>
> 
> This is problematic, since with this signature you should be able to
> create this piece of code:
> 
>     let lock: SpinLockIrq = /* ... */
> 
>     let mut guard = None;
>     with_irq_disabled(|irq| guard = Some(lock.lock_with(irq)));
>     // then use guard when `irq` are again enabled!
> 
> So what we want the signature to be is this:
>     
>     pub fn lock_with<'a>(&'a self, _context: B::Context<'a>) -> Guard<'a, T, B>
> 
> So we need to ensure that the lifetime of `context` is bound to the
> lifetime of the guard.

Gotcha
> 
> >          // 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..327e53be4c0f4 100644
> > --- a/rust/kernel/sync/lock/mutex.rs
> > +++ b/rust/kernel/sync/lock/mutex.rs
> > @@ -4,6 +4,8 @@
> >  //!
> >  //! This module allows Rust code to use the kernel's `struct mutex`.
> > 
> > +use core::marker::*;
> > +
> >  /// Creates a [`Mutex`] initialiser with the given name and a newly-created lock class.
> >  ///
> >  /// It uses the name if one is given, otherwise it generates one based on the file name and line
> > @@ -93,6 +95,7 @@ macro_rules! new_mutex {
> >  unsafe impl super::Backend for MutexBackend {
> >      type State = bindings::mutex;
> >      type GuardState = ();
> > +    type Context<'a> = PhantomData<&'a ()>;
> 
> Is there any reason you chose `PhantomData` here? Why not just `()`?

Oh! I think I had just naturally assumed that if an associated type had a
lifetime declared that its contents had to use said lifetime, but I guess
that's not actually the case :). Will fix this

> 
> ---
> Cheers,
> Benno
> 
> > 
> >      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..8503d6e8e3de3 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> = PhantomData<&'a ()>;
> > 
> >      unsafe fn init(
> >          ptr: *mut Self::State,
> > --
> > 2.45.2
> > 
> 

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

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