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 BBB071A9FA4; Sat, 18 Jul 2026 05:33:04 +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=1784352786; cv=none; b=D2Rw8/r2yZIclbIy0rrDNHz33E+y+K82e66EaSWge+KEET9xvAj/ktub7Y7XWw7WmpRq67o1LV/uYHcWu5c/FKLt4Jehws9oDHXHfbb3tN1hkkTGH231OeOlBEbotQPBL07IYgzbsfXafO7t4cQKmpALvM8SOV1AMElOMQi/O1M= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784352786; c=relaxed/simple; bh=TNn/jDEr9CbQuxOMUVdYM2YyDJveN3eRBuNASoulf4M=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=sE4pkh2FHMPc7lw+yRVX8AdG7qGPSlGNYYcdkttmww8rglqu5nxqGD1VNUF1tNSX8NwQd9xfxRAJ/ZaAZO0r8Ax1yOLTwjJkQbhXNRjckb8dXp+LK5bETXUFeXd3WtkfvS5wJiiHitM+JHg4jD28RFjSMM7t3RkdTxm79uT2rKU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=fU1ozw21; 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="fU1ozw21" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5FA951F00AC4; Sat, 18 Jul 2026 05:33:03 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784352784; bh=YumMbTuFN39udjMu2dZ9HB8drw55p7kF/M9QUGWthnc=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=fU1ozw21g3OiLiw32hofDBp594Bnbsf4fggg3vRf2B3N20vySW14GDyXM3Zd/55Ok BmJK5lEN2a99FJFR76Qi3qYkDMDW3cHPMF8Bvj8jMoQV9iSH4/6LBnBLYjyOC79N5R xJMGDMUaJLqd4falmtUhigxSUtYOVjqdrtuzz+7aYaGkqC1atbUT8mFPrGJ6UE2Y0z RxaOO2ZGETbyVDjQTdX9lP7YHALngdas1wuaWSXYQ2PeB39r8ikeVge1fjNQ8Uw7L3 VuX21L+Cz0hlaj+NgKXsQ29XGBGKGJ5NeRxFeU9ygADtXeSGVUXNqV51RZqQOduFFk SaAbYGWIhwReg== Received: from phl-compute-05.internal (phl-compute-05.internal [10.202.2.45]) by mailfauth.phl.internal (Postfix) with ESMTP id 910F2F4006A; Sat, 18 Jul 2026 01:33:02 -0400 (EDT) Received: from phl-frontend-03 ([10.202.2.162]) by phl-compute-05.internal (MEProxy); Sat, 18 Jul 2026 01:33:02 -0400 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: dmFkZTEe1tEuSFJRyejgcADWwC1Q31VcIulMbzlwxQxSwM7p7NELQtXxoL4t9u3P3h6szL wb7v4Y1Nfd9PJf44pRYto2n4BU3w63GmouT4LNR6cMb94hByhC4T19UClHqFpzwzazj5Vf A8YkKW1UUR0TJ8dV49ki0VbHb8uMKM6KId0g8EWioE2qFZQpDctdLDeip2hFqPq5d3ACH5 qO7/M64UwGhfkVp49Uw1mz7RUEmunGbHxCq20xcQsFHR4VpiWV6KRY8BvJYxsbWtxpbsKA SwMJCFL53Iue2y6cUSY74fyEj9Wo66A5FNIdAV9MLOMsDNkZ9FUjwGTU+R2pAhU2dl3BiV 3I6CxXPrsaR7ob/+XwDdwE1bYBvmRiUsRk7/rUs+UYDvdKLKB7I6Eu1kIAIIDcE1+a4imU YHcJetBuL/vOAIOYwH6aaoJDy+GJIhenv+zw/dHzAIgAqqJYpd3e5O78/+ODcs2txRksBs Ewqmdqb5NZoKU++D/70ThHKygiJvh5+z3zklLGIVDfh1OjUh01ut04C68MFuC2qz3KMABj JuDPLErNsJpKDu8I1saGiCQEKmgO8R0Vw8A3yJBeComoPpSEVZJI3K6KXgBdSdFOGDn2KY d8uzNvMiS6CBun5S29KMsO3+5Is6UApvD+5wrf+PGpBpTm2rIsuZVK74ylLQ X-ME-Proxy: Feedback-ID: i8dbe485b:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Sat, 18 Jul 2026 01:33:01 -0400 (EDT) From: Boqun Feng To: 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 , Daniel Almeida , Tamir Duberstein , Alexandre Courbot , =?UTF-8?q?Onur=20=C3=96zkan?= , "Liam R. Howlett" , Andrew Ballance , Alexander Viro , Christian Brauner , Jan Kara , Lyude Paul , "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?= , Greg Kroah-Hartman , "Yury Norov (NVIDIA)" , Asahi Lina , Matthew Maurer , Lorenzo Stoakes , linux-kernel@vger.kernel.org, maple-tree@lists.infradead.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-media@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org Subject: [RFC PATCH v2 3/4] rust: rcu: Introduce RcuFreeBox Date: Fri, 17 Jul 2026 22:32:46 -0700 Message-ID: <20260718053247.25154-4-boqun@kernel.org> X-Mailer: git-send-email 2.50.1 In-Reply-To: <20260718053247.25154-1-boqun@kernel.org> References: <20260718053247.25154-1-boqun@kernel.org> Precedence: bulk X-Mailing-List: linux-fsdevel@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 | 34 ++++++++++++++++ rust/kernel/sync/rcu/rcu_box.rs | 72 +++++++++++++++++++++++++++++++-- 2 files changed, 102 insertions(+), 4 deletions(-) diff --git a/rust/kernel/sync/rcu.rs b/rust/kernel/sync/rcu.rs index 42f6bbc83f71..e781a5044de9 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::{ @@ -18,6 +20,8 @@ pub use self::rcu_box::RcuKVBox; pub use self::rcu_box::RcuVBox; +pub use self::rcu_box::RcuFreeBox; + /// Evidence that the RCU read side lock is held on the current thread/CPU. /// /// The type is explicitly not `Send` because this property is per-thread/CPU. @@ -100,3 +104,33 @@ 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 { + /// Clean up `Self` and make it ready to be RCU freed. + 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 cb1fd422480a..ab4e9b2a4444 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. @@ -223,6 +225,56 @@ 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)?)) + } +} + +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 transparent to `RcuBoxInner`, and `RcuBox` + // owns the object per type invariants. + let inner: *mut RcuBoxInner = self.0 .0.as_ptr().cast(); + + // SAFETY: Per the invariants of `RcuBox`, `inner` owns the pointed object. And we are not + // going to move it. + let pin = unsafe { Pin::new_unchecked(&mut (*inner).value) }; + + pin.drop_before_gp(); + + // `needs_drop::()` returns `false`, hence `kvfree_call_rcu()` will be called + // and free the underlying data after a grace period. + } +} + +// Note that `T: Sync` is required since when moving an `RcuFreeBox`, the previous owner may +// still access `&T` for one grace period. +// +// SAFETY: Ownership of the `RcuFreeBox` allows for `&T` and dropping the `T`, so `T: Send + +// Sync` implies `RcuFreeBox: Send`. +unsafe impl Send for RcuFreeBox {} + +// SAFETY: `&RcuFreeBox` allows for no operations other than those permitted by `&T`, so `T: +// Sync` implies `RcuFreeBox: Sync`. +unsafe impl Sync for RcuFreeBox {} + #[kunit_tests(rust_rcu_box)] mod tests { use super::*; @@ -236,6 +288,12 @@ fn rcu_box_basic() -> Result { drop(rb); + let rb = RcuFreeBox::<_, alloc::allocator::Kmalloc>::new(42i32, alloc::flags::GFP_KERNEL)?; + + assert_eq!(*rb, 42); + + drop(rb); + let rb = RcuBox::<_, alloc::allocator::Vmalloc>::new(42i32, alloc::flags::GFP_KERNEL)?; assert_eq!(*rb, 42); @@ -243,6 +301,12 @@ fn rcu_box_basic() -> Result { drop(rb); + let rb = RcuFreeBox::<_, alloc::allocator::Vmalloc>::new(42i32, alloc::flags::GFP_KERNEL)?; + + assert_eq!(*rb, 42); + + drop(rb); + Ok(()) } } -- 2.50.1 (Apple Git-155)