Re: [PATCH 2/3] rust: sync: Introduce LockContainer trait

[Lyude Paul <lyude@redhat.com>]

On Fri, 2024-07-26 at 07:40 +0000, Benno Lossin wrote:
> On 26.07.24 00:27, Lyude Paul wrote:
> > We want to be able to use spinlocks in no-interrupt contexts, but our
> > current `Lock` infrastructure doesn't allow for the ability to pass
> > arguments when acquiring a lock - meaning that there would be no way for us
> > to verify interrupts are disabled before granting a lock since we have
> > nowhere to pass an `IrqGuard`.
> > 
> > It doesn't particularly made sense for us to add the ability to pass such
> > an argument either: this would technically work, but then we would have to
> > pass empty units as arguments on all of the many locks that are not grabbed
> > under interrupts. As a result, we go with a slightly nicer solution:
> 
> I think there is a solution that would allow us to have both[1]:
> 1. Add a new associated type to `Backend` called `Context`.
> 2. Add a new parameter to `Backend::lock`: `ctx: Self::Context`.
> 3. Add a new function to `Lock<T: ?Sized, B: Backend>`:
>    `lock_with(&self, ctx: B::Context)` that delegates to `B::lock`.
> 4. Reimplement `Lock::lock` in terms of `Lock::lock_with`, by
>    constraining the function to only be callable if
>    `B::Context: Default` holds (and then using `Default::default()` as
>    the value).
> 
> This way people can still use `lock()` as usual, but we can also have
> `lock_with(irq)` for locks that require it.

ooo! I like this idea :), this totally sounds good to me and I'll do this in
the next iteration of patches

> 
> [1]: I think I saw this kind of a pattern first from Wedson in the
> context of passing default allocation flags.
> 
> > introducing a trait for types which can contain a lock of a specific type:
> > LockContainer. This means we can still use locks implemented on top of
> > other lock types in types such as `LockedBy` - as we convert `LockedBy` to
> > begin using `LockContainer` internally and implement the trait for all
> > existing lock types.
> 
> 
> > 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> > ---
> >  rust/kernel/sync.rs           |  1 +
> >  rust/kernel/sync/lock.rs      | 20 ++++++++++++++++++++
> >  rust/kernel/sync/locked_by.rs | 11 +++++++++--
> >  3 files changed, 30 insertions(+), 2 deletions(-)
> > 
> > diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
> > index 0ab20975a3b5d..14a79ebbb42d5 100644
> > --- a/rust/kernel/sync.rs
> > +++ b/rust/kernel/sync.rs
> > @@ -16,6 +16,7 @@
> >  pub use condvar::{new_condvar, CondVar, CondVarTimeoutResult};
> >  pub use lock::mutex::{new_mutex, Mutex};
> >  pub use lock::spinlock::{new_spinlock, SpinLock};
> > +pub use lock::LockContainer;
> >  pub use locked_by::LockedBy;
> > 
> >  /// Represents a lockdep class. It's a wrapper around C's `lock_class_key`.
> > diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs
> > index f6c34ca4d819f..bbd0a7465cae3 100644
> > --- a/rust/kernel/sync/lock.rs
> > +++ b/rust/kernel/sync/lock.rs
> > @@ -195,3 +195,23 @@ pub(crate) unsafe fn new(lock: &'a Lock<T, B>, state: B::GuardState) -> Self {
> >          }
> >      }
> >  }
> > +
> > +/// A trait implemented by any type which contains a [`Lock`] with a specific [`Backend`].
> > +pub trait LockContainer<T: ?Sized, B: Backend> {
> > +    /// Returns an immutable reference to the lock
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// Since this returns a reference to the contained [`Lock`] without going through the
> > +    /// [`LockContainer`] implementor, it cannot be guaranteed that it is safe to acquire
> > +    /// this lock. Thus the caller must promise not to attempt to use the returned immutable
> > +    /// reference to attempt to grab the underlying lock without ensuring whatever guarantees the
> > +    /// [`LockContainer`] implementor's interface enforces.
> 
> This safety requirement is rather unclear to me, there isn't really a
> good place to put the `LockContainer` requirements when implementing
> this trait.
> I also don't understand the use-case where a lock can only be acquired
> in certain circumstances, do you have an example?
> 
> ---
> Cheers,
> Benno
> 
> > +    unsafe fn get_lock_ref(&self) -> &Lock<T, B>;
> > +}
> > +
> > +impl<T: ?Sized, B: Backend> LockContainer<T, B> for Lock<T, B> {
> > +    unsafe fn get_lock_ref(&self) -> &Lock<T, B> {
> > +        &self
> > +    }
> > +}
> > diff --git a/rust/kernel/sync/locked_by.rs b/rust/kernel/sync/locked_by.rs
> > index babc731bd5f62..d16d89fe74e0b 100644
> > --- a/rust/kernel/sync/locked_by.rs
> > +++ b/rust/kernel/sync/locked_by.rs
> > @@ -95,13 +95,20 @@ impl<T, U> LockedBy<T, U> {
> >      /// data becomes inaccessible; if another instance of the owner is allocated *on the same
> >      /// memory location*, the data becomes accessible again: none of this affects memory safety
> >      /// because in any case at most one thread (or CPU) can access the protected data at a time.
> > -    pub fn new<B: Backend>(owner: &Lock<U, B>, data: T) -> Self {
> > +    pub fn new<B, L>(owner: &L, data: T) -> Self
> > +    where
> > +        B: Backend,
> > +        L: super::LockContainer<U, B>,
> > +    {
> >          build_assert!(
> >              size_of::<Lock<U, B>>() > 0,
> >              "The lock type cannot be a ZST because it may be impossible to distinguish instances"
> >          );
> >          Self {
> > -            owner: owner.data.get(),
> > +            // SAFETY: We never directly acquire the lock through this reference, we simply use it
> > +            // to ensure that a `Guard` the user provides us to access this container's contents
> > +            // belongs to the same lock that owns this data
> > +            owner: unsafe { owner.get_lock_ref() }.data.get(),
> >              data: UnsafeCell::new(data),
> >          }
> >      }
> > --
> > 2.45.2
> > 
> 

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

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