From: Gary Guo Date: Fri, 5 Sep 2025 04:41:39 +0000 (-0700) Subject: rust: convert `Arc` to use `Refcount` X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=076acb647c1f448177d8b3b0e4f33de959713d7d;p=thirdparty%2Fkernel%2Fstable.git rust: convert `Arc` to use `Refcount` With `Refcount` type created, `Arc` can use `Refcount` instead of calling into FFI directly. Signed-off-by: Gary Guo Signed-off-by: Boqun Feng Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Alexandre Courbot Reviewed-by: Benno Lossin Reviewed-by: Elle Rhumsaa Link: https://lore.kernel.org/r/20250723233312.3304339-4-gary@kernel.org --- diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs index 4ee155b43b2dc..9298993ea7d8b 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. @@ -18,11 +18,11 @@ use crate::{ alloc::{AllocError, Flags, KBox}, - bindings, ffi::c_void, init::InPlaceInit, + sync::Refcount, try_init, - types::{ForeignOwnable, Opaque}, + types::ForeignOwnable, }; use core::{ alloc::Layout, @@ -145,7 +145,7 @@ pub struct Arc { #[pin_data] #[repr(C)] struct ArcInner { - refcount: Opaque, + refcount: Refcount, data: T, } @@ -157,7 +157,7 @@ impl ArcInner { /// `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> { - let refcount_layout = Layout::new::(); + let refcount_layout = Layout::new::(); // 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 @@ -229,8 +229,7 @@ impl Arc { pub fn new(contents: T, flags: Flags) -> Result { // 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, }; @@ -348,18 +347,13 @@ impl Arc { // We will manually manage the refcount in this method, so we disable the destructor. let this = ManuallyDrop::new(this); // SAFETY: We own a refcount, so the pointer is still valid. - let refcount = unsafe { this.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); // 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 @@ -456,14 +450,10 @@ impl Borrow for Arc { impl Clone for Arc { 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) } @@ -472,16 +462,10 @@ impl Clone for Arc { impl Drop for Arc { 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. + let is_zero = unsafe { self.ptr.as_ref() }.refcount.dec_and_test(); if is_zero { // The count reached zero, we must free the memory. // @@ -775,8 +759,7 @@ impl UniqueArc { // INVARIANT: The refcount is initialised to a non-zero value. let inner = KBox::try_init::( 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 <- pin_init::uninit::(), }? AllocError), flags,