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 20CDD24DCE3; Thu, 24 Jul 2025 00:00:19 +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=1753315221; cv=none; b=ODEEkIlPJdRrM5n++uzT91IbJQ+tQzLsDPZZo3qocw2xgjsLzuXPpVpdPPka5bb47Ck4gndUk+zEFDFuJ96B9zvAhmSneceNN55zxs6BAnrVrZmD6rg7yK/E15deTH7ByVbxM/KerdRSlpr6Tx1QG5EO82HMJHXSZeSPEaj6eb4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1753315221; c=relaxed/simple; bh=TMO1q/yEQRaEVcz1Gj8Of2O2/owEkXMVj9dfWg7k7FI=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=Y/jfltoBwiqVqVaJ/JKI7mkZ6MmwyUz4S7nI7Eco0Hiq+ByFiPb8C5M7FHtCs6FQVeGlXyERhP6fsMTOOzIbyOGBOazhd89P02t3ASeG7+NjR8KolTvWOpPiBEIsGzncR35Jqy9kvNTtKdUx/OY1/KnVp7jhCz5HlP/0b4WPHXY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=vJdXS4qY; 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="vJdXS4qY" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7EB32C4CEEF; Thu, 24 Jul 2025 00:00:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1753315219; bh=TMO1q/yEQRaEVcz1Gj8Of2O2/owEkXMVj9dfWg7k7FI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=vJdXS4qYNrnsr8tUJ37NQb3l1bAsU16Jdm3hd8zSg9Q8zItzzKtziIaS1f0Wv42un YkxA80wyWzCkJUM6n6VZP04XH/NRQeKflouthLyzbACAikxNOrHiS4HPFMstc6yo9I vELOoPegx9wZ9kjdY1ah/Hnw7UDqooMrnuzlUPH5fwn+lKUaLXXBk7m65dnskCy80l UZ9c8Yuar/rYplRu1TLeFwJmb/10BHbTiwX5UwvFZT0sAYifHezFkgwccWhVUQQPsR RS5hw7FXHFp2omUc4qgcGAXnJXBhxob5bG8uYYiGNGg4dFiR861lMS8C8dOQYuuRyT YqzWtWoMB2BVw== 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 , Tamir Duberstein , Alexandre Courbot , Alex Mantel Cc: Will Deacon , Peter Zijlstra , Mark Rutland , rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH v5 3/5] rust: convert `Arc` to use `Refcount` Date: Thu, 24 Jul 2025 00:32:56 +0100 Message-ID: <20250723233312.3304339-4-gary@kernel.org> X-Mailer: git-send-email 2.49.0 In-Reply-To: <20250723233312.3304339-1-gary@kernel.org> References: <20250723233312.3304339-1-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 +++++++++++++---------------------------- 1 file changed, 14 insertions(+), 31 deletions(-) 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 @@ 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 @@ -456,14 +450,10 @@ fn borrow(&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) } @@ -472,16 +462,10 @@ 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. // @@ -775,8 +759,7 @@ 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, -- 2.49.0