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* Re: [PATCH v2] rust: adding UniqueRefCounted and UniqueRef types
@ 2025-02-28 18:22 Oliver Mangold
  2025-02-28 18:33 ` Boqun Feng
  0 siblings, 1 reply; 14+ messages in thread
From: Oliver Mangold @ 2025-02-28 18:22 UTC (permalink / raw)
  To: Boqun Feng
  Cc: Andreas Hindborg, Miguel Ojeda, Alex Gaynor, Gary Guo,
	Björn Roy Baron, Benno Lossin, Alice Ryhl, Trevor Gross,
	linux-kernel, rust-for-linux

On 250228 1009, Boqun Feng wrote:
> 
> Please add the "# Examples" section as I requested:
> 
> 	https://lore.kernel.org/rust-for-linux/Z8HcHVtPiG-X6ujP@Mac.home/
> 
> I also would like to know why do you think it's OK to ignore my previous
> comment, thanks!
> 

I intend to. Don't worry. I totally have it in mind that, this is missing.
As said, I will do it in a couple of days. Sorry if the update
caused confusion.

I am aware this won't be the final version, but I thought
I still post this one correctly formatted, as Miguel requested.

I assure you, I'm not ignoring your comment and request at all.

Oliver


^ permalink raw reply	[flat|nested] 14+ messages in thread
* Re: [PATCH v2] rust: adding UniqueRefCounted and UniqueRef types
@ 2025-02-28 18:41 Oliver Mangold
  0 siblings, 0 replies; 14+ messages in thread
From: Oliver Mangold @ 2025-02-28 18:41 UTC (permalink / raw)
  To: Andreas Hindborg
  Cc: Miguel Ojeda, Alex Gaynor, Boqun Feng, Gary Guo,
	Björn Roy Baron, Benno Lossin, Alice Ryhl, Trevor Gross,
	linux-kernel, rust-for-linux

On 250228 1929, Andreas Hindborg wrote:
> 
> I would suggest splitting out the use case from the commit message:
> 
Yes, that makes sense. Will do.
> 
> The comments after the --- will not got in the commit log, they are a
> message for the review process only.
> 
Ahhh. Honestly I'm not very familiar with the format-patch/am
method, yet. That was a gap in my understanding.
Thanks for pointing that out. Now everything makes sense.

On 250228 1033, Boqun Feng wrote:
> 
> Now as we cleared things, I want to make sure that it's clear that I
> would like to see the patch with examples in it.
>
That's fine. That's a reasonable request. Will do.
> 
> i.e. you can add some description after the "---" line, and that won't
> affect git to apply the patches. Usually people put changes between
> versions, and in this case since you just want to update a version that
> is not the final version, you can add some description about that there
> as well.
> 
Yes. I just learned that it's working like this. See above :)
> 
> Thanks! Again, sorry for not being clear in previous emails.
>
Don't worry.


Best,

Oliver


^ permalink raw reply	[flat|nested] 14+ messages in thread
* [PATCH] rust: adding UniqueRefCounted and UniqueRef types
@ 2025-02-28 14:43 Oliver Mangold
  2025-02-28 18:01 ` [PATCH v2] " Oliver Mangold
  0 siblings, 1 reply; 14+ messages in thread
From: Oliver Mangold @ 2025-02-28 14:43 UTC (permalink / raw)
  To: Andreas Hindborg
  Cc: Miguel Ojeda, Alex Gaynor, Boqun Feng, Gary Guo,
	Björn Roy Baron, Benno Lossin, Alice Ryhl, Trevor Gross,
	linux-kernel, rust-for-linux

Hi,

For usage with block-mq, we found that having a variant of ARef
which is guaranteed to be unique being useful.
As chances are it is useful in general, I implemented it
as kernel::types::UniqueRef.
The difference between ARef and UniqueRef
is basically the same as between Arc and UniqueArc.

This v2 of the patch, addressing the issues raised by Andreas Hindborg.

On 250228 1417, Miguel Ojeda wrote:
> 
> I think should be caught by Clippy -- Oliver, please try building with
> `CLIPPY=1` (we would like Clippy-clean builds as much as reasonably
> possible),

Got it. This version should be okay for rustfmt, clippy and checkpatch :)

Best regards,

Oliver

---
 rust/kernel/types.rs | 153 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 153 insertions(+)

diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
index 55ddd50e8aaa..72a973d9e1c7 100644
--- a/rust/kernel/types.rs
+++ b/rust/kernel/types.rs
@@ -543,6 +543,12 @@ fn from(b: &T) -> Self {
     }
 }
 
+impl<T: UniqueRefCounted> From<UniqueRef<T>> for ARef<T> {
+    fn from(b: UniqueRef<T>) -> Self {
+        UniqueRefCounted::unique_to_shared(b)
+    }
+}
+
 impl<T: AlwaysRefCounted> Drop for ARef<T> {
     fn drop(&mut self) {
         // SAFETY: The type invariants guarantee that the `ARef` owns the reference we're about to
@@ -551,6 +557,153 @@ fn drop(&mut self) {
     }
 }
 
+/// Types that are [`AlwaysRefCounted`] and can be safely converted to an [`UniqueRef`]
+///
+/// # Safety
+///
+/// Implementers must ensure that the methods of the trait
+/// change the reference count of the underlying object such that:
+/// - the uniqueness invariant is upheld, i.e. it is not possible
+///   to obtain another reference by any means (other than through the [`UniqueRef`])
+///   until the [`UniqueRef`] is dropped or converted to an [`ARef`].
+/// - [`UniqueRefCounted::dec_ref`] correctly frees the underlying object.
+/// - [`UniqueRefCounted::unique_to_shared`] set the reference count to the value
+/// - that the returned [`ARef`] expects for an object with a single reference
+///   in existence.
+pub unsafe trait UniqueRefCounted: AlwaysRefCounted + Sized {
+    /// Checks if the [`ARef`] is unique and convert it
+    /// to an [`UniqueRef`] it that is that case.
+    /// Otherwise it returns again an [`ARef`] to the same
+    /// underlying object.
+    fn try_shared_to_unique(this: ARef<Self>) -> Result<UniqueRef<Self>, ARef<Self>>;
+    /// Converts the [`UniqueRef`] into an [`ARef`].
+    fn unique_to_shared(this: UniqueRef<Self>) -> ARef<Self>;
+    /// Decrements the reference count on the object when the [`UniqueRef`] is dropped.
+    ///
+    /// Frees the object when the count reaches zero.
+    ///
+    /// It defaults to [`AlwaysRefCounted::dec_ref`],
+    /// but overriding it may be useful, e.g. in case of non-standard refcounting
+    /// schemes.
+    ///
+    /// # Safety
+    ///
+    /// The same safety constraints as for [`AlwaysRefCounted::dec_ref`] apply,
+    /// but as the reference is unique, it can be assumed that the function
+    /// will not be called twice. In case the default implementation is not
+    /// overridden, it has to be ensured that the call to [`AlwaysRefCounted::dec_ref`]
+    /// can be used for an [`UniqueRef`], too.
+    unsafe fn dec_ref(obj: NonNull<Self>) {
+        // SAFETY: correct by function safety requirements
+        unsafe { AlwaysRefCounted::dec_ref(obj) };
+    }
+}
+
+/// An unique, owned reference to an [`AlwaysRefCounted`] object.
+///
+/// It works the same ways as [`ARef`] but ensures that the reference is unique
+/// and thus can be dereferenced mutably.
+///
+/// # Invariants
+///
+/// - The pointer stored in `ptr` is non-null and valid for the lifetime of the [`UniqueRef`]
+///   instance. In particular, the [`UniqueRef`] instance owns an increment
+///   on the underlying object's reference count.
+/// - No other references to the underlying object exist while the [`UniqueRef`] is live.
+pub struct UniqueRef<T: UniqueRefCounted> {
+    ptr: NonNull<T>,
+    _p: PhantomData<T>,
+}
+
+// SAFETY: It is safe to send `UniqueRef<T>` to another thread
+// when the underlying `T` is `Sync` because
+// it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
+// `T` to be `Send` because any thread that has an `UniqueRef<T>` may ultimately access `T` using a
+// mutable reference, for example, when the reference count reaches zero and `T` is dropped.
+unsafe impl<T: UniqueRefCounted + Sync + Send> Send for UniqueRef<T> {}
+
+// SAFETY: It is safe to send `&UniqueRef<T>` to another thread when the underlying `T` is `Sync`
+// because it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally,
+// it needs `T` to be `Send` because any thread that has a `&UniqueRef<T>` may clone it and get an
+// `UniqueRef<T>` on that thread, so the thread may ultimately access `T`
+// using a mutable reference, for example, when the reference count reaches zero and `T` is dropped.
+unsafe impl<T: UniqueRefCounted + Sync + Send> Sync for UniqueRef<T> {}
+
+impl<T: UniqueRefCounted> UniqueRef<T> {
+    /// Creates a new instance of [`UniqueRef`].
+    ///
+    /// It takes over an increment of the reference count on the underlying object.
+    ///
+    /// # Safety
+    ///
+    /// Callers must ensure that the reference count is set to such a value
+    /// that a call to [`UniqueRefCounted::dec_ref`] will release the underlying object
+    /// in the way which is expected when the last reference is dropped.
+    /// Callers must not use the underlying object anymore --
+    /// it is only safe to do so via the newly created [`UniqueRef`].
+    pub unsafe fn from_raw(ptr: NonNull<T>) -> Self {
+        // INVARIANT: The safety requirements guarantee that the new instance now owns the
+        // increment on the refcount.
+        Self {
+            ptr,
+            _p: PhantomData,
+        }
+    }
+
+    /// Consumes the [`UniqueRef`], returning a raw pointer.
+    ///
+    /// This function does not change the refcount. After calling this function, the caller is
+    /// responsible for the refcount previously managed by the [`UniqueRef`].
+    pub fn into_raw(me: Self) -> NonNull<T> {
+        ManuallyDrop::new(me).ptr
+    }
+}
+
+impl<T: UniqueRefCounted> Deref for UniqueRef<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: The type invariants guarantee that the object is valid.
+        unsafe { self.ptr.as_ref() }
+    }
+}
+
+impl<T: UniqueRefCounted> DerefMut for UniqueRef<T> {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        // SAFETY: The type invariants guarantee that the object is valid.
+        unsafe { self.ptr.as_mut() }
+    }
+}
+
+impl<T: UniqueRefCounted> From<&T> for UniqueRef<T> {
+    /// Converts the [`UniqueRef`] into an [`ARef`]
+    /// by calling [`UniqueRefCounted::unique_to_shared`] on it.
+    fn from(b: &T) -> Self {
+        b.inc_ref();
+        // SAFETY: We just incremented the refcount above.
+        unsafe { Self::from_raw(NonNull::from(b)) }
+    }
+}
+
+impl<T: UniqueRefCounted> TryFrom<ARef<T>> for UniqueRef<T> {
+    type Error = ARef<T>;
+    /// Tries to convert the [`ARef`] to an [`UniqueRef`]
+    /// by calling [`UniqueRefCounted::try_shared_to_unique`].
+    /// In case the [`ARef`] is not unique it returns again an [`ARef`] to the same
+    /// underlying object.
+    fn try_from(b: ARef<T>) -> Result<UniqueRef<T>, Self::Error> {
+        UniqueRefCounted::try_shared_to_unique(b)
+    }
+}
+
+impl<T: UniqueRefCounted> Drop for UniqueRef<T> {
+    fn drop(&mut self) {
+        // SAFETY: The type invariants guarantee that the [`UniqueRef`] owns the reference
+        // we're about to decrement.
+        unsafe { UniqueRefCounted::dec_ref(self.ptr) };
+    }
+}
+
 /// A sum type that always holds either a value of type `L` or `R`.
 ///
 /// # Examples
-- 
2.48.1

Signed-off-by: Oliver Mangold <oliver.mangold@pm.me>


^ permalink raw reply related	[flat|nested] 14+ messages in thread
end of thread, other threads:[~2025-03-03 14:10 UTC | newest]

Thread overview: 14+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2025-02-28 18:22 [PATCH v2] rust: adding UniqueRefCounted and UniqueRef types Oliver Mangold
2025-02-28 18:33 ` Boqun Feng
2025-02-28 19:01   ` Andreas Hindborg
2025-02-28 19:06     ` Boqun Feng
2025-02-28 19:16       ` Oliver Mangold
2025-02-28 19:25         ` Andreas Hindborg
2025-02-28 19:47           ` Oliver Mangold
2025-02-28 20:07           ` Oliver Mangold
  -- strict thread matches above, loose matches on Subject: below --
2025-02-28 18:41 Oliver Mangold
2025-02-28 14:43 [PATCH] " Oliver Mangold
2025-02-28 18:01 ` [PATCH v2] " Oliver Mangold
2025-02-28 18:09   ` Boqun Feng
2025-02-28 18:16     ` Boqun Feng
2025-02-28 18:29   ` Andreas Hindborg
2025-03-03 14:09   ` Andreas Hindborg

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