From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 67EB531B101; Fri, 5 Jun 2026 13:35:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780666560; cv=none; b=QuLGY6kWBBnL1bAaSyWFKVZbBPKwkCQS8R7A3TMFiEOIDi7/GNRbNl0RUa/S9i1bs7tjkMYEyn8Sg21RC2+baAa1r44f78f8uK7YN94TJXthua+mi+s/tAVk42mRboiF1aOhMOEtL/MSPDNx7yWmPwDK5B/bJgudaSJk/kSYTFc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780666560; c=relaxed/simple; bh=tEimvk7OykDDRIUJs/A3HLeKRmTl3/7VQfcmhS4wJFM=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=BS9frXli6M65SC1F7pMO5j0a3PfGZONbeoaFsr9Y+xpWNINC1O81fMOHq875sBGC2zIc2LwmAfeucQtIzA4iRuRP/6k0ZmJREiKk0SlWllERCCrvOVoEsKN18MMfVXNt3vxERj0nAJgvaV1mHrrkvjcuBUoKld0KciU/IztBAIc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=lXVXqKQU; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="lXVXqKQU" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5FF2D1F0089E; Fri, 5 Jun 2026 13:35:56 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780666557; bh=3KosL0D3Q2eEjvojKVK/tB6tB4IEDd35FarHmws6fUs=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=lXVXqKQU/sP/FRbSGZUw52acvlgIharWh19NaZUaft+jpptgVn7wpE+l1Py6LbksH 5yW9iK06yCralSUxHMWDfgKlmDZyubI6ImpHprLY0j4elpQCyEuw/GDHfv5YdmvhSc 6YICmB+FjlBC2e5GHGz7uViHP8MCXuRunpGroN6HKOo4emRSL8mj3znxf8spKEIASW lRjM9FYzCbmSmFPW6nW2Jpyqn619h5P15VJVsTeQyqkj7vc1+tK8SCpBTz2lsKnvVw 0C9YUz5wZ0diAnzZRNCY6cTFwLFg6WDDVhfbvLWO12unifOEjP1oYtNp5NZBUF3NBI o+v2fwOmPz1jw== Received: from phl-compute-06.internal (phl-compute-06.internal [10.202.2.46]) by mailfauth.phl.internal (Postfix) with ESMTP id AED08F4006F; Fri, 5 Jun 2026 09:35:55 -0400 (EDT) Received: from phl-frontend-04 ([10.202.2.163]) by phl-compute-06.internal (MEProxy); Fri, 05 Jun 2026 09:35:55 -0400 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: dmFkZTEiXmK80I+r4b+5zY/r99GUAlO/A1JeBVvrzdPJND7AxnXROcGIdrV5+IW/GIICbk HTeSUzo2jG7T3ubuXDYawvMsJmw7SeRzXaVjmYhKpesaWDi/jHlBwDSw7cUJtOL4Zml+HR MdH91GjghnCvkbpZdNzjjo1bSZDmeAE+i20ut4ugSeq7SenIcQ9DZOfghRt4xUUc5uh5PM SgAN181ZxF1oco7PoJ9ho3wb1kPOXsEzQT5UZv0CFFx1JIqpGEiANmmsdwoALURinC3es0 +e8I9nc2247wfagtNBtbgyVV8BvBRc03DGsTzzDlNTrxNmxxQ9+IH7k1Xg1Flh6mzaY9AF F1Rbx4zjkpU7Vu9L15KSs/58k3UtTFycBLbXnKzW7G3Az8oIg8YJ7B1llLRzq9ZQymYw+g 2fPji44Zl+dmnKR9uYe/gkEuH7gJ9937pSdsAM+jHiIhPy6/Q3Ya1wrMQnGQgK/yBtLMlz YUObbhrUwbZHNvplmabTTmM68ir7kNT02nLZMMPNC7GcsuPZ3JhN9G4NGFO8U2/zbN2LhN qlb41qjjYJSqfOr+cv1dH5tLNyYc8aMoVJZIMB9KUDldh4Ho9IlsJ2x/ZYZJl0fSmk1QXV FZ7uk0R3q0t0R5NcBwbJCLKAGRtG8d3QOaxly+IElSGTUJZLozO/dAinufEw X-ME-Proxy: Feedback-ID: i8dbe485b:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Fri, 5 Jun 2026 09:35:54 -0400 (EDT) From: Boqun Feng To: linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, rcu@vger.kernel.org Cc: Miguel Ojeda , Boqun Feng , Gary Guo , =?UTF-8?q?Bj=C3=B6rn=20Roy=20Baron?= , Benno Lossin , Andreas Hindborg , Alice Ryhl , Trevor Gross , Danilo Krummrich , "Liam R. Howlett" , Andrew Ballance , "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang , Sumit Semwal , =?UTF-8?q?Christian=20K=C3=B6nig?= , maple-tree@lists.infradead.org, linux-mm@kvack.org, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, Philipp Stanner , Lyude Paul , Daniel Almeida , =?UTF-8?q?Onur=20=C3=96zkan?= Subject: [RFC PATCH 3/3] rust: rcu: Introduce RcuFreeBox Date: Fri, 5 Jun 2026 06:35:41 -0700 Message-ID: <20260605133541.22569-6-boqun@kernel.org> X-Mailer: git-send-email 2.51.0 In-Reply-To: <20260605133541.22569-1-boqun@kernel.org> References: <20260605133541.22569-1-boqun@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 The current RcuBox will call the `drop()` function after a grace period inside an RCU callback. This suffices for maintaining a RCU-protected object: RcuBox::drop(): call_rcu( |..| { // <- call back after one grace period. T::drop(); // <- call the destructor of the inner object. } ) However, to support a different RCU usage pattern as below we need to extend RcuBox: 1. clean up the object, and unshare it from future RCU readers. 2. wait for an RCU grace period. 3. no other RCU readers, we can free the memory. An `RcuFreeBox` is introduced to provide support for this: RcuFreeBox::drop(): T::drop_before_gp(); // clean up and ushare. kfree_call_rcu(..); // free it after one grace period. Signed-off-by: Boqun Feng --- rust/kernel/sync/rcu.rs | 31 +++++++++++++++ rust/kernel/sync/rcu/rcu_box.rs | 68 +++++++++++++++++++++++++++++++-- 2 files changed, 95 insertions(+), 4 deletions(-) diff --git a/rust/kernel/sync/rcu.rs b/rust/kernel/sync/rcu.rs index 7da6b8d22277..7c26591bb318 100644 --- a/rust/kernel/sync/rcu.rs +++ b/rust/kernel/sync/rcu.rs @@ -4,6 +4,8 @@ //! //! C header: [`include/linux/rcupdate.h`](srctree/include/linux/rcupdate.h) +use core::pin::Pin; + use crate::{ bindings, types::{ @@ -82,3 +84,32 @@ pub trait ForeignOwnableRcu: ForeignOwnable { /// [`from_foreign`]: ForeignOwnable::from_foreign unsafe fn rcu_borrow<'a>(ptr: *mut ffi::c_void) -> Self::RcuBorrowed<'a>; } + +/// Declares a struct is safe to free after a grace period if all readers are guarded by RCU. +/// +/// # Safety +/// +/// Implementation must guarantee `drop_before_gp()` makes sure no future RCU reader will access +/// any part of [`Self`], as a result, after `drop_before_gp()` return + one grace period, no RCU +/// reader will be on the object, and it's safe to free it. +/// +/// Notes for implementators: implementing this trait in general requires `Self` being a +/// [`UnsafePinned`], i.e. a `&mut Self` is not a noalias reference if `Self` has non-trivial +/// `drop()` function. +pub unsafe trait RcuFreeSafe { + fn drop_before_gp(self: Pin<&mut Self>); +} + +macro_rules! impl_not_drop { + ($($t:ty, )*) => { + // SAFETY: Dropping `T` has no side effect means `T` is always ready to be freed. And an + // empty `drop_before_gp()` suffices. + $(unsafe impl RcuFreeSafe for $t { + fn drop_before_gp(self: Pin<&mut Self>) { + $crate::const_assert!(!core::mem::needs_drop::<$t>()); + } + })* + } +} + +impl_not_drop! {i8,u8,i16,u16,i32,u32,isize,usize,i64,u64,} diff --git a/rust/kernel/sync/rcu/rcu_box.rs b/rust/kernel/sync/rcu/rcu_box.rs index 943fe3e8974e..8f52bb472daf 100644 --- a/rust/kernel/sync/rcu/rcu_box.rs +++ b/rust/kernel/sync/rcu/rcu_box.rs @@ -6,6 +6,7 @@ use core::{ marker::PhantomData, + mem::ManuallyDrop, ops::Deref, ptr::NonNull, // }; @@ -29,17 +30,18 @@ use super::{ ForeignOwnableRcu, - Guard, // + Guard, + RcuFreeSafe, // }; -/// A box that is freed with rcu. +/// A box that is drop with RCU. /// -/// The value must be `Send`, as rcu may drop it on another thread. +/// The value must be `Send`, as RCU may drop it on another thread. /// /// # Invariants /// /// * The pointer is valid and references a pinned `RcuBoxInner` allocated with `A`. -/// * This `RcuBox` holds exclusive permissions to rcu free the allocation. +/// * This `RcuBox` holds exclusive permissions to RCU-free the allocation. pub struct RcuBox(NonNull>, PhantomData); /// Type alias for [`RcuBox`] with a [`Kmalloc`] allocator. @@ -205,6 +207,50 @@ fn drop(&mut self) { drop(unsafe { Box::<_, A>::from_raw(box_inner) }); } +/// A box that is freed with RCU. +/// +/// Currently we require `T` being `Send` because of an implementation limitation. In theory we can +/// support `T` being `!Send`, since the RCU callback is only used to free the memory, not dropping +/// `T`. +pub struct RcuFreeBox(RcuBox, A>); + +impl RcuFreeBox { + /// Create a new `RcuFreeBox`. + pub fn new(x: T, flags: alloc::Flags) -> Result { + Ok(Self(RcuBox::new(ManuallyDrop::new(x), flags)?)) + } + + /// Access the value for a grace period. + pub fn with_rcu<'rcu>(&self, read_guard: &'rcu Guard) -> &'rcu T { + self.0.with_rcu(read_guard) + } +} + +impl Deref for RcuFreeBox { + type Target = T; + + fn deref(&self) -> &T { + self.0.deref() + } +} + +impl Drop for RcuFreeBox { + fn drop(&mut self) { + // CAST: `ManuallyDrop` is transparet to `T`, adn `RcuBox` owns the object per type + // invariants. + let ptr = self.0 .0.as_ptr().cast::(); + + // SAFETY: Per the invariants of `RcuBox`, `ptr` owns the pointed object. And we are not + // going to move it. + let pin = unsafe { Pin::new_unchecked(&mut *ptr) }; + + pin.drop_before_gp(); + + // `needs_drop::()` returns `false`, hence `kvfree_call_rcu()` will be called + // and free the underlying data after a gracer period. + } +} + #[kunit_tests(rust_rcu_box)] mod tests { use super::*; @@ -218,6 +264,13 @@ fn rcu_box_basic() -> Result { drop(rb); + let rb = RcuFreeBox::<_, alloc::allocator::Kmalloc>::new(42i32, alloc::flags::GFP_KERNEL)?; + + assert_eq!(*rb, 42); + assert_eq!(*rb.with_rcu(&Guard::new()), 42); + + drop(rb); + let rb = RcuBox::<_, alloc::allocator::Vmalloc>::new(42i32, alloc::flags::GFP_KERNEL)?; assert_eq!(*rb, 42); @@ -225,6 +278,13 @@ fn rcu_box_basic() -> Result { drop(rb); + let rb = RcuFreeBox::<_, alloc::allocator::Vmalloc>::new(42i32, alloc::flags::GFP_KERNEL)?; + + assert_eq!(*rb, 42); + assert_eq!(*rb.with_rcu(&Guard::new()), 42); + + drop(rb); + Ok(()) } } -- 2.51.0