From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2AE5ABA34; Sun, 22 Jun 2025 12:59:01 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750597142; cv=none; b=CWy/U6HOzp6Kjc+BXYemJUHSCOFeylXSqEHY5bQURX3LpyqsDfQmJBXffHurSSuLn52q9YdAV/+s0eZ2urlhN2h+2G1112KnegNUsdo+DrhTVSfPf8Y5mgNoBtbsu7IFBuvY6GYOkBiHvmSkkd30RbYRLWoJfZwgTor/ACOQaTk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750597142; c=relaxed/simple; bh=psmatb5q2y4XNrP55GujTA94WAv8VwfGnVaR5bowiek=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=jrzMkPAsAhRw1l8HOjoCBC6cSAsD1+aUR/wN5uBXdyc/hkxcjTFE2dhrt5RG8Jeq8pBjyCjLq7D6rYM6LjwKI8C6Ir/LmHvtFsgyOpIvrfqN9u+3o+npU0dJ1K3FtQffWk4WTQL6Oue1b6A2A7N4NvPXO4VRg31XDZVWbdf7rdw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=jGKgFRsq; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="jGKgFRsq" Received: by smtp.kernel.org (Postfix) with ESMTPSA id ABD21C4CEE3; Sun, 22 Jun 2025 12:58:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1750597141; bh=psmatb5q2y4XNrP55GujTA94WAv8VwfGnVaR5bowiek=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=jGKgFRsq97zoKzeFZc0y0omJL5h+rRF2eofmE48FopeeJEN/cIopdFTaC/BayxU3X LCCLXKLPSVVV473J6CNJKSQ21fK5vIf5hB02HkUg1+xy9c+g5uvJMO3EFRDSwX2euO jl4YvO3dZOXxk0PqQ3OxABR8GQufust2IbPmPaF5/Q7lTxIueilpRyCG1WqheVwRMf 9NBHQw/RfdTNIgfCJzPNaV+pr7vmi8uKkEt/YrkNmRhVoA1hAAkCk+x4Ze7OT79c1X DUFPD5IfWWPmnb2U3nf8vaHszLCe5HRUbQaU2F0XaL+Ob2pKa13UTruuLyIsEUjTlC +oOB43aUofT0A== From: Gary Guo To: Miguel Ojeda , Alex Gaynor , Boqun Feng , Gary Guo , =?UTF-8?q?Bj=C3=B6rn=20Roy=20Baron?= , Benno Lossin , Andreas Hindborg , Alice Ryhl , Trevor Gross , Danilo Krummrich , Will Deacon , Peter Zijlstra , Mark Rutland , Tamir Duberstein , Xiangfei Ding , Alex Mantel Cc: rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v4 3/5] rust: convert `Arc` to use `Refcount` Date: Sun, 22 Jun 2025 13:57:29 +0100 Message-ID: <20250622125802.3224264-4-gary@kernel.org> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20250622125802.3224264-1-gary@kernel.org> References: <20250622125802.3224264-1-gary@kernel.org> Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Gary Guo With `Refcount` type created, `Arc` can use `Refcount` instead of calling into FFI directly. Signed-off-by: Gary Guo --- rust/kernel/sync/arc.rs | 45 +++++++++++------------------------- rust/kernel/sync/refcount.rs | 8 +++++++ 2 files changed, 22 insertions(+), 31 deletions(-) diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs index 09a471fea4da..fc63ded00f6e 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::InPlaceInit, + sync::Refcount, try_init, - types::{ForeignOwnable, Opaque}, + types::ForeignOwnable, }; use core::{ alloc::Layout, @@ -144,7 +144,7 @@ pub struct Arc { #[pin_data] #[repr(C)] pub struct ArcInner { - refcount: Opaque, + refcount: Refcount, data: T, } @@ -156,7 +156,7 @@ 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 @@ -228,8 +228,7 @@ 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, }; @@ -347,18 +346,13 @@ pub fn into_unique_or_drop(this: Self) -> Option>> { // 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 @@ -428,14 +422,10 @@ fn as_ref(&self) -> &T { 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) } @@ -444,16 +434,10 @@ fn clone(&self) -> Self { 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. // @@ -747,8 +731,7 @@ pub fn new_uninit(flags: Flags) -> Result>, AllocError> // 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, diff --git a/rust/kernel/sync/refcount.rs b/rust/kernel/sync/refcount.rs index a0fc22f6d645..8e7b9b0c1979 100644 --- a/rust/kernel/sync/refcount.rs +++ b/rust/kernel/sync/refcount.rs @@ -71,6 +71,14 @@ pub fn dec(&self) { /// must come after. /// /// Returns true if the resulting refcount is 0, false otherwise. + /// + /// # Notes + /// + /// A common pattern of using `Refcount` is to free memory when the reference count reaches + /// zero. This means that the reference to `Refcount` could become invalid after calling this + /// function. This is fine as long as the reference to `Refcount` is no longer used when this + /// function returns `false`. It is not necessary to use raw pointers in this scenario, see + /// https://github.com/rust-lang/rust/issues/55005. #[inline] #[must_use = "use `dec` instead if you do not need to test if it is 0"] pub fn dec_and_test(&self) -> bool { -- 2.49.0