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 B89A53E2764; Wed, 20 May 2026 13:59:47 +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=1779285589; cv=none; b=J34h0bdBncfGhbqQ+82DnVqJR87WzpyIunKu6J0+vlv97q3c9TjZhr2RHYMOVe7TYMYTrianyJ/yGXsMVODAg6/5A64X9jx06kVvQaRxFb81qNz/8gZMEqUEJShpUcU6eV3pILsDjAGkrGpsT6F0z6stytCmWTY/SzGFPX7ZTY0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779285589; c=relaxed/simple; bh=2pGSacfZAIqQprf1Lf6i8EJO5A34sYZYKccrsCtmGLU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=mpqOgosIzL8Q2qVUJHecHPoDsOA53DqHL9WwoDei6t2YNWLgYvLp0lcdqridDY1Rjvn+rOMZfrjz66LVHBUtUu2V2ONdu/8cVuIM7Y+Yo+yEV/90POFu3Y/wVsndxUaFHiItSc0TE5PPWq6ySFx/FrbI+Pf+HkuhdRmgO5kW4uE= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=n1UotxEo; 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="n1UotxEo" Received: by smtp.kernel.org (Postfix) with ESMTPSA id CB7DC1F000E9; Wed, 20 May 2026 13:59:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1779285586; bh=+01xROXyws5N5mI8/Iz3HZAtwQAchJYV8+RRZQTxSNc=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=n1UotxEoSLaRztUzY84QT4fE/79WysYzlXgD8VBLApRvVf8hZdmn44UPhsiffvt6Q ZddisGYunR7cj9wxDscdXsr1qf5S54Bz4GpyE41sLOlTUrWFeR1gl24K6bIPiShjme vx51n7Va2p4l0Ce3jSU6k9vvA6E+UNfNeJtiFYxzZcs7gQFYtrxrwdQie0oVnuY0fO RdSHxTj9ZHJVq9yDDB3tMLYZVjK9MP0TlLWmjzlVnDpsdozq9FfwWd43o5bJoZN5an r/ZzQ0USTaRP2NTCQpyNtc2BzO8HeFPyVtYHLsEhvY3LvEDdQOszlCpvMnlpzayaZq ota61yaAW/NpQ== Received: from phl-compute-06.internal (phl-compute-06.internal [10.202.2.46]) by mailfauth.phl.internal (Postfix) with ESMTP id 2DD6EF40075; Wed, 20 May 2026 09:59:45 -0400 (EDT) Received: from phl-frontend-04 ([10.202.2.163]) by phl-compute-06.internal (MEProxy); Wed, 20 May 2026 09:59:45 -0400 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgeefhedrtddtgddugeegkeduucetufdoteggodetrf dotffvucfrrhhofhhilhgvmecuhfgrshhtofgrihhlpdfurfetoffkrfgpnffqhgenuceu rghilhhouhhtmecufedttdenucesvcftvggtihhpihgvnhhtshculddquddttddmnecujf gurhepfffhvfevuffkfhggtggujgesthdtredttddtvdenucfhrhhomhepuehoqhhunhcu hfgvnhhguceosghoqhhunheskhgvrhhnvghlrdhorhhgqeenucggtffrrghtthgvrhhnpe elueehtefhtddtgfejvdejueehhfekteevueeuueekgeetieeggeehvdffhefhhfenucff ohhmrghinhepkhgvrhhnvghlrdhorhhgnecuvehluhhsthgvrhfuihiivgeptdenucfrrg hrrghmpehmrghilhhfrhhomhepsghoqhhunhdomhgvshhmthhprghuthhhphgvrhhsohhn rghlihhthidqudeijedtleekgeejuddqudejjeekheehhedvqdgsohhquhhnpeepkhgvrh hnvghlrdhorhhgsehfihigmhgvrdhnrghmvgdpnhgspghrtghpthhtohepvdeipdhmohgu vgepshhmthhpohhuthdprhgtphhtthhopehphhgrshhtrgeskhgvrhhnvghlrdhorhhgpd hrtghpthhtohepohhjvggurgeskhgvrhhnvghlrdhorhhgpdhrtghpthhtohepghgrrhih sehgrghrhihguhhordhnvghtpdhrtghpthhtohepsghjohhrnhefpghghhesphhrohhtoh hnmhgrihhlrdgtohhmpdhrtghpthhtoheplhhoshhsihhnsehkvghrnhgvlhdrohhrghdp rhgtphhtthhopegrrdhhihhnuggsohhrgheskhgvrhhnvghlrdhorhhgpdhrtghpthhtoh eprghlihgtvghrhihhlhesghhoohhglhgvrdgtohhmpdhrtghpthhtohepthhmghhrohhs shesuhhmihgthhdrvgguuhdprhgtphhtthhopegurghkrheskhgvrhhnvghlrdhorhhg X-ME-Proxy: Feedback-ID: i8dbe485b:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Wed, 20 May 2026 09:59:44 -0400 (EDT) Date: Wed, 20 May 2026 06:59:43 -0700 From: Boqun Feng To: Philipp Stanner Cc: Miguel Ojeda , Gary Guo , =?iso-8859-1?Q?Bj=F6rn?= Roy Baron , Benno Lossin , Andreas Hindborg , Alice Ryhl , Trevor Gross , Danilo Krummrich , "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , Zqiang , "Joel Fernandes (NVIDIA)" , "Peter Zijlstra (Intel)" , Tamir Duberstein , rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, rcu@vger.kernel.org Subject: Re: [PATCH v1] rust: rcu: Add abstraction for call_rcu() Message-ID: References: <20260520131725.266014-2-phasta@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260520131725.266014-2-phasta@kernel.org> Hi Philipp, On Wed, May 20, 2026 at 03:17:26PM +0200, Philipp Stanner wrote: > call_rcu() can be expected to be needed by a great variety of users. > This functionality is almost always used for deallocating resources > after all accessors are gone. Hence, it appears reasonable to implement > the abstractions in such a way that the user merely passes data, which > is later (after a grace period) dropped. > > In the rare cases where the user needs special action to take place, > this could be achieved through implementing a custom drop() method. > > Implement a first minimal abstraction for call_rcu(). > Thanks for the patch! Do you have have any reference usage of this new API, maybe contains how RCU readers will read the data? Compared to Alice's RcuBox proposal: https://lore.kernel.org/rust-for-linux/20260116-rcu-box-v1-1-38ebfbcd53f0@google.com/ I do have a design question: is support data type like Arc> or Pin> in the plan of this API? If so, how would that be like? A separate new() and submit() function or a separate data type? If not, what's the main difference between Callback API and RcuBox? Regards, Boqun > Signed-off-by: Philipp Stanner > --- > rust/helpers/rcu.c | 1 + > rust/kernel/sync.rs | 1 + > rust/kernel/sync/rcu.rs | 89 ++++++++++++++++++++++++++++++++++++++++- > 3 files changed, 90 insertions(+), 1 deletion(-) > > diff --git a/rust/helpers/rcu.c b/rust/helpers/rcu.c > index 481274c05857..c9cfc99c93d5 100644 > --- a/rust/helpers/rcu.c > +++ b/rust/helpers/rcu.c > @@ -1,5 +1,6 @@ > // SPDX-License-Identifier: GPL-2.0 > > +#include /* for callback_head */ > #include > > __rust_helper void rust_helper_rcu_read_lock(void) > diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs > index 993dbf2caa0e..1ddca3847b19 100644 > --- a/rust/kernel/sync.rs > +++ b/rust/kernel/sync.rs > @@ -31,6 +31,7 @@ > pub use locked_by::LockedBy; > pub use refcount::Refcount; > pub use set_once::SetOnce; > +pub use rcu::Callback; > > /// Represents a lockdep class. > /// > diff --git a/rust/kernel/sync/rcu.rs b/rust/kernel/sync/rcu.rs > index a32bef6e490b..caf71fa46f5e 100644 > --- a/rust/kernel/sync/rcu.rs > +++ b/rust/kernel/sync/rcu.rs > @@ -4,7 +4,15 @@ > //! > //! C header: [`include/linux/rcupdate.h`](srctree/include/linux/rcupdate.h) > > -use crate::{bindings, types::NotThreadSafe}; > +use crate::{ > + bindings, > + prelude::*, > + types::{ > + NotThreadSafe, > + Opaque, > + }, > + alloc::Flags, > +}; > > /// Evidence that the RCU read side lock is held on the current thread/CPU. > /// > @@ -50,3 +58,82 @@ fn drop(&mut self) { > pub fn read_lock() -> Guard { > Guard::new() > } > + > + > +/// An RCU callback object. Carries the user's data to drop() it once a grace period ellapsed. > +/// > +/// This object serves to implement C's `call_rcu()` method. Since it is almost > +/// always used to free a resource once a grace period ellapsed, the only thing > +/// this implementation does is drop the user's data. In the rare cases in which > +/// the user needs more action to take place, said actions need to be implemented > +/// on the user's data via the [`Drop`] trait. > +/// > +/// # Examples > +/// > +/// ``` > +/// use kernel::sync::rcu::Callback; > +/// > +/// struct Foo {}; > +/// > +/// impl Drop for Foo { > +/// fn drop(&mut self) { > +/// pr_info!("rcu::Foo Dropping.\n"); > +/// } > +/// } > +/// > +/// let data = Foo {}; > +/// > +/// let cb = Callback::new(data, GFP_KERNEL)?; > +/// cb.submit(); > +/// > +/// Ok::<(), Error>(()) > +/// ``` > +#[repr(C)] > +#[pin_data] > +pub struct Callback { > + /// The RCU head. Only used (and initialized) by the C backend. > + #[pin] > + inner: Opaque, > + /// The user's data. This should implement [`Drop`] if the user wants specific > + /// actions, besides mere deallocation, to happen. > + #[pin] > + data: T, > +} > + > +impl Callback { > + /// Create a new callback. > + pub fn new(data: impl PinInit, flags: Flags) -> Result>> { > + let cb = try_pin_init!(Self { > + inner: Opaque::uninit(), // Only needed for the C backend, who will initialize it. > + data <- data, > + }); > + > + KBox::pin_init(cb, flags) > + } > + > + extern "C" fn callback(rcu_head: *mut bindings::callback_head) { > + let cb_ptr = rcu_head as *mut Self; > + > + // SAFETY: All [`Callback`] objects in this module are always created > + // as `Pin>`. `Pin` is a transparent container. The action > + // below merely serves re-creating the KBox so that it can drop properly. > + let _cb = unsafe { KBox::from_raw(cb_ptr) }; > + > + // Self::data drops, ensuring the desired cleanup operation. > + } > + > + fn as_raw(&self) -> *mut bindings::callback_head { > + self.inner.get() > + } > + > + /// Arm a [`Callback`]. One grace period after this function was called, > + /// the callback object will be dropped. > + pub fn submit(self: Pin>) { > + // SAFETY: The memory is not moved by this code or the C backend. > + let cb = unsafe { Pin::into_inner_unchecked(self) }; > + let ptr = KBox::into_raw(cb); > + // SAFETY: `ptr` was just created validly above. `Self::callback` relies > + // on the RCU module / code never being unloaded. > + unsafe { bindings::call_rcu((*ptr).as_raw(), Some(Self::callback)) }; > + } > +} > -- > 2.49.0 >