Messages in this thread Patch in this message |  | | Date | Sat, 31 Dec 2022 19:43:52 +0000 | From | Gary Guo <> | Subject | Re: [PATCH 4/7] rust: sync: introduce `ArcBorrow` |
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On Wed, 28 Dec 2022 06:03:43 +0000 Wedson Almeida Filho <wedsonaf@gmail.com> wrote:
> This allows us to create references to a ref-counted allocation without > double-indirection and that still allow us to increment the refcount to > a new `Arc<T>`. > > Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com> > --- > rust/kernel/sync.rs | 2 +- > rust/kernel/sync/arc.rs | 97 +++++++++++++++++++++++++++++++++++++++++ > 2 files changed, 98 insertions(+), 1 deletion(-) > > diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs > index 39b379dd548f..5de03ea83ea1 100644 > --- a/rust/kernel/sync.rs > +++ b/rust/kernel/sync.rs > @@ -7,4 +7,4 @@ > > mod arc; > > -pub use arc::Arc; > +pub use arc::{Arc, ArcBorrow}; > diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs > index dbc7596cc3ce..f68bfc02c81a 100644 > --- a/rust/kernel/sync/arc.rs > +++ b/rust/kernel/sync/arc.rs > @@ -19,6 +19,7 @@ use crate::{bindings, error::Result, types::Opaque}; > use alloc::boxed::Box; > use core::{ > marker::{PhantomData, Unsize}, > + mem::ManuallyDrop, > ops::Deref, > ptr::NonNull, > }; > @@ -164,6 +165,18 @@ impl<T: ?Sized> Arc<T> { > _p: PhantomData, > } > } > + > + /// Returns an [`ArcBorrow`] from the given [`Arc`]. > + /// > + /// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method > + /// receiver), but we have an [`Arc`] instead. Getting an [`ArcBorrow`] is free when optimised. > + #[inline] > + pub fn as_arc_borrow(&self) -> ArcBorrow<'_, T> { > + // SAFETY: The constraint that the lifetime of the shared reference must outlive that of > + // the returned `ArcBorrow` ensures that the object remains alive and that no mutable > + // reference can be created. > + unsafe { ArcBorrow::new(self.ptr) } > + } > } > > impl<T: ?Sized> Deref for Arc<T> { > @@ -208,3 +221,87 @@ impl<T: ?Sized> Drop for Arc<T> { > } > } > } > + > +/// A borrowed reference to an [`Arc`] instance. > +/// > +/// For cases when one doesn't ever need to increment the refcount on the allocation, it is simpler > +/// to use just `&T`, which we can trivially get from an `Arc<T>` instance. > +/// > +/// However, when one may need to increment the refcount, it is preferable to use an `ArcBorrow<T>` > +/// over `&Arc<T>` because the latter results in a double-indirection: a pointer (shared reference) > +/// to a pointer (`Arc<T>`) to the object (`T`). An [`ArcBorrow`] eliminates this double > +/// indirection while still allowing one to increment the refcount and getting an `Arc<T>` when/if > +/// needed. > +/// > +/// # Invariants > +/// > +/// There are no mutable references to the underlying [`Arc`], and it remains valid for the > +/// lifetime of the [`ArcBorrow`] instance. > +/// > +/// # Example > +/// > +/// ``` > +/// use crate::sync::{Arc, ArcBorrow}; > +/// > +/// struct Example; > +/// > +/// fn do_something(e: ArcBorrow<'_, Example>) -> Arc<Example> { > +/// e.into() > +/// } > +/// > +/// let obj = Arc::try_new(Example)?; > +/// let cloned = do_something(obj.as_arc_borrow()); > +/// > +/// // Assert that both `obj` and `cloned` point to the same underlying object. > +/// assert!(core::ptr::eq(&*obj, &*cloned)); > +/// ``` > +pub struct ArcBorrow<'a, T: ?Sized + 'a> { > + inner: NonNull<ArcInner<T>>, > + _p: PhantomData<&'a ()>, > +} > + > +impl<T: ?Sized> Clone for ArcBorrow<'_, T> { > + fn clone(&self) -> Self { > + *self > + } > +} > + > +impl<T: ?Sized> Copy for ArcBorrow<'_, T> {}
Couldn't this just be derived `Clone` and `Copy`?
> + > +impl<T: ?Sized> ArcBorrow<'_, T> { > + /// Creates a new [`ArcBorrow`] instance. > + /// > + /// # Safety > + /// > + /// Callers must ensure the following for the lifetime of the returned [`ArcBorrow`] instance: > + /// 1. That `inner` remains valid; > + /// 2. That no mutable references to `inner` are created. > + unsafe fn new(inner: NonNull<ArcInner<T>>) -> Self { > + // INVARIANT: The safety requirements guarantee the invariants. > + Self { > + inner, > + _p: PhantomData, > + } > + } > +} > + > +impl<T: ?Sized> From<ArcBorrow<'_, T>> for Arc<T> { > + fn from(b: ArcBorrow<'_, T>) -> Self { > + // SAFETY: The existence of `b` guarantees that the refcount is non-zero. `ManuallyDrop` > + // guarantees that `drop` isn't called, so it's ok that the temporary `Arc` doesn't own the > + // increment. > + ManuallyDrop::new(unsafe { Arc::from_inner(b.inner) }) > + .deref() > + .clone() > + } > +}
It might be easier to follow if this is jsut `bindings::refcount_inc` followed by `Arc::from_inner`? > + > +impl<T: ?Sized> Deref for ArcBorrow<'_, T> { > + type Target = T; > + > + fn deref(&self) -> &Self::Target { > + // SAFETY: By the type invariant, the underlying object is still alive with no mutable > + // references to it, so it is safe to create a shared reference. > + unsafe { &self.inner.as_ref().data } > + } > +}
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