Message-ID: <CAJ-ks9npk8oSFHZHdViR1XhF+A8e2L+P0wCgmjE7mzAxS9WK1g@mail.gmail.com>
Date: Wed, 19 Feb 2025 17:12:10 -0500
From: Tamir Duberstein <tamird@...il.com>
To: Gary Guo <gary@...yguo.net>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, Wedson Almeida Filho <walmeida@...rosoft.com>, 
	Alex Mantel <alexmantel93@...lbox.org>, Will Deacon <will@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Mark Rutland <mark.rutland@....com>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/4] rust: convert `Arc` to use `Refcount`

On Wed, Feb 19, 2025 at 3:17 PM Gary Guo <gary@...yguo.net> wrote:
>
> With `Refcount` type created, `Arc` can use `Refcount` instead of
> calling into FFI directly.
>
> Signed-off-by: Gary Guo <gary@...yguo.net>
> ---
>  rust/kernel/sync/arc.rs | 65 +++++++++++++++++------------------------
>  1 file changed, 26 insertions(+), 39 deletions(-)
>
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index 3cefda7a4372..1f5fbc6b3742 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -8,7 +8,7 @@
>  //! threads.
>  //!
>  //! It is different from the standard library's [`Arc`] in a few ways:
> -//! 1. It is backed by the kernel's `refcount_t` type.
> +//! 1. It is backed by the kernel's [`Refcount`] type.
>  //! 2. It does not support weak references, which allows it to be half the size.
>  //! 3. It saturates the reference count instead of aborting when it goes over a threshold.
>  //! 4. It does not provide a `get_mut` method, so the ref counted object is pinned.
> @@ -18,10 +18,10 @@
>
>  use crate::{
>      alloc::{AllocError, Flags, KBox},
> -    bindings,
>      init::{self, InPlaceInit, Init, PinInit},
> +    sync::Refcount,
>      try_init,
> -    types::{ForeignOwnable, Opaque},
> +    types::ForeignOwnable,
>  };
>  use core::{
>      alloc::Layout,
> @@ -143,7 +143,7 @@ pub struct Arc<T: ?Sized> {
>  #[pin_data]
>  #[repr(C)]
>  struct ArcInner<T: ?Sized> {
> -    refcount: Opaque<bindings::refcount_t>,
> +    refcount: Refcount,
>      data: T,
>  }
>
> @@ -155,7 +155,7 @@ impl<T: ?Sized> ArcInner<T> {
>      /// `ptr` must have been returned by a previous call to [`Arc::into_raw`], and the `Arc` must
>      /// not yet have been destroyed.
>      unsafe fn container_of(ptr: *const T) -> NonNull<ArcInner<T>> {
> -        let refcount_layout = Layout::new::<bindings::refcount_t>();
> +        let refcount_layout = Layout::new::<Refcount>();
>          // SAFETY: The caller guarantees that the pointer is valid.
>          let val_layout = Layout::for_value(unsafe { &*ptr });
>          // SAFETY: We're computing the layout of a real struct that existed when compiling this
> @@ -207,8 +207,7 @@ impl<T> Arc<T> {
>      pub fn new(contents: T, flags: Flags) -> Result<Self, AllocError> {
>          // INVARIANT: The refcount is initialised to a non-zero value.
>          let value = ArcInner {
> -            // SAFETY: There are no safety requirements for this FFI call.
> -            refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
> +            refcount: Refcount::new(1),
>              data: contents,
>          };
>
> @@ -290,7 +289,7 @@ pub fn ptr_eq(this: &Self, other: &Self) -> bool {
>      /// use kernel::sync::{Arc, UniqueArc};
>      ///
>      /// let arc = Arc::new(42, GFP_KERNEL)?;
> -    /// let unique_arc = arc.into_unique_or_drop();
> +    /// let unique_arc = Arc::into_unique_or_drop(arc);
>      ///
>      /// // The above conversion should succeed since refcount of `arc` is 1.
>      /// assert!(unique_arc.is_some());
> @@ -306,35 +305,30 @@ pub fn ptr_eq(this: &Self, other: &Self) -> bool {
>      /// let arc = Arc::new(42, GFP_KERNEL)?;
>      /// let another = arc.clone();
>      ///
> -    /// let unique_arc = arc.into_unique_or_drop();
> +    /// let unique_arc = Arc::into_unique_or_drop(arc);
>      ///
>      /// // The above conversion should fail since refcount of `arc` is >1.
>      /// assert!(unique_arc.is_none());
>      ///
>      /// # Ok::<(), Error>(())
>      /// ```
> -    pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
> +    pub fn into_unique_or_drop(this: Self) -> Option<Pin<UniqueArc<T>>> {

Why did this signature need to change?

>          // We will manually manage the refcount in this method, so we disable the destructor.
> -        let me = ManuallyDrop::new(self);
> +        let this = ManuallyDrop::new(this);
>          // SAFETY: We own a refcount, so the pointer is still valid.
> -        let refcount = unsafe { me.ptr.as_ref() }.refcount.get();
> +        let refcount = unsafe { &this.ptr.as_ref().refcount };
>
>          // If the refcount reaches a non-zero value, then we have destroyed this `Arc` and will
>          // return without further touching the `Arc`. If the refcount reaches zero, then there are
>          // no other arcs, and we can create a `UniqueArc`.
> -        //
> -        // SAFETY: We own a refcount, so the pointer is not dangling.
> -        let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
> -        if is_zero {
> -            // SAFETY: We have exclusive access to the arc, so we can perform unsynchronized
> -            // accesses to the refcount.
> -            unsafe { core::ptr::write(refcount, bindings::REFCOUNT_INIT(1)) };
> +        if refcount.dec_and_test() {
> +            refcount.set(1);

We could retain the unsynchronized operation here by taking a mutable
reference above and writing through it. Right? Could we remove `set`
from the abstraction in the previous patch?

>
> -            // INVARIANT: We own the only refcount to this arc, so we may create a `UniqueArc`. We
> -            // must pin the `UniqueArc` because the values was previously in an `Arc`, and they pin
> -            // their values.
> +            // INVARIANT: If the refcount failed to decrement because it is 1, then we have the
> +            // exclusive ownership, so we may create a `UniqueArc`. We must pin the `UniqueArc`
> +            // because the values was previously in an `Arc`, and they pin their values.

Pre-existing typo you're taking ownership of: "the values" should be
"the value". But why touch this comment at all?

>              Some(Pin::from(UniqueArc {
> -                inner: ManuallyDrop::into_inner(me),
> +                inner: ManuallyDrop::into_inner(this),
>              }))
>          } else {
>              None
> @@ -396,14 +390,10 @@ fn as_ref(&self) -> &T {
>
>  impl<T: ?Sized> Clone for Arc<T> {
>      fn clone(&self) -> Self {
> -        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
> -        // safe to dereference it.
> -        let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
> -
> -        // INVARIANT: C `refcount_inc` saturates the refcount, so it cannot overflow to zero.
> +        // INVARIANT: `Refcount` saturates the refcount, so it cannot overflow to zero.
>          // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
>          // safe to increment the refcount.
> -        unsafe { bindings::refcount_inc(refcount) };
> +        unsafe { self.ptr.as_ref().refcount.inc() };
>
>          // SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`.
>          unsafe { Self::from_inner(self.ptr) }
> @@ -412,16 +402,14 @@ fn clone(&self) -> Self {
>
>  impl<T: ?Sized> Drop for Arc<T> {
>      fn drop(&mut self) {
> -        // SAFETY: By the type invariant, there is necessarily a reference to the object. We cannot
> -        // touch `refcount` after it's decremented to a non-zero value because another thread/CPU
> -        // may concurrently decrement it to zero and free it. It is ok to have a raw pointer to
> -        // freed/invalid memory as long as it is never dereferenced.
> -        let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
> -
>          // INVARIANT: If the refcount reaches zero, there are no other instances of `Arc`, and
>          // this instance is being dropped, so the broken invariant is not observable.
> -        // SAFETY: Also by the type invariant, we are allowed to decrement the refcount.
> -        let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
> +        // SAFETY: By the type invariant, there is necessarily a reference to the object.
> +        // NOTE: we cannot touch `refcount` after it's decremented to a non-zero value because
> +        // another thread/CPU may concurrently decrement it to zero and free it. However it is okay
> +        // to have a transient reference to decrement the refcount, see
> +        // https://github.com/rust-lang/rust/issues/55005.
> +        let is_zero = unsafe { self.ptr.as_ref().refcount.dec_and_test() };

How come this careful handling is not required in into_unique_or_drop?
At least, the SAFETY comment there is much more mundane.

>          if is_zero {
>              // The count reached zero, we must free the memory.
>              //
> @@ -673,8 +661,7 @@ pub fn new_uninit(flags: Flags) -> Result<UniqueArc<MaybeUninit<T>>, AllocError>
>          // INVARIANT: The refcount is initialised to a non-zero value.
>          let inner = KBox::try_init::<AllocError>(
>              try_init!(ArcInner {
> -                // SAFETY: There are no safety requirements for this FFI call.
> -                refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
> +                refcount: Refcount::new(1),
>                  data <- init::uninit::<T, AllocError>(),
>              }? AllocError),
>              flags,
> --
> 2.47.2
>

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