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 043971ACEBB; Wed, 18 Dec 2024 12:38:50 +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=1734525531; cv=none; b=qx/QZAH/ECL0EZr/a/SpZ/HzOL/gWz1kNZEtGrwIqALQtKsy9Gdr/BxptNnws/zKU7JK6vgnUgfISeJBLyxBeYaz7BQgom+cJFvVHVoC+QZ+/XynhZ+KMrjleY5gGVtVHYgo3gAr7efiTtDpUJ4H79eM73xB6wc6nrjUfLF8MbI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1734525531; c=relaxed/simple; bh=2OTsU3J7r7LPomFBxDI1YLlstwgCJhxCK3/OJyvPlqc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=JEdYikpaKhd0OAAQnktUmdYTuxtrvkBJjFBRyQyff4ecBBQMBljd0Bh9+pGZP0lyKxje2DNqjHdddQyRmbs449hNwzhCoiPJUH+Wd/MDvKsFDk9q7yDRIwVfwQTh9qoNmiNeidZyTAz/qU9NHjmQeF2TchTMQIxKRBl7mm1eyD8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=LsecNBgL; 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="LsecNBgL" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 5FDD8C4CECE; Wed, 18 Dec 2024 12:38:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1734525530; bh=2OTsU3J7r7LPomFBxDI1YLlstwgCJhxCK3/OJyvPlqc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=LsecNBgLDQ5LlWd5JhkivZx18dnOSf+IX1sKq+MD3kL7mXr6kuTGxDlwdVUito5eo 7CnSomWbWw/c0Vd5AvMh//eNicJPycWSC4/oL0ZQYP0vxsL+QmdJDXxCQcJkmO7QZZ 9fT1o6iAaIx1ryuXJJfhz0aN2HzxS2Cp1m5fnxsxMiJWHwqVMOyeZWoi+bLSS4s8Q8 DIlfUOEcNNddaC4alrbWxLMMIdIvEDVnYxV9py7LhtGHMKQIkA7UrYfxANkR0NQs0o mDi84lBa797/pV7apnqbvfbQWLwAB/KxgtboC1rcs7AFzFOcYntzJ/khxF+6TJx/Bn 7NyRrP5uX4/ag== Date: Wed, 18 Dec 2024 13:38:41 +0100 From: Danilo Krummrich To: =?iso-8859-1?Q?Beno=EEt?= du Garreau Cc: gregkh@linuxfoundation.org, rafael@kernel.org, bhelgaas@google.com, ojeda@kernel.org, alex.gaynor@gmail.com, boqun.feng@gmail.com, gary@garyguo.net, bjorn3_gh@protonmail.com, benno.lossin@proton.me, tmgross@umich.edu, a.hindborg@samsung.com, aliceryhl@google.com, airlied@gmail.com, fujita.tomonori@gmail.com, lina@asahilina.net, pstanner@redhat.com, ajanulgu@redhat.com, lyude@redhat.com, robh@kernel.org, daniel.almeida@collabora.com, saravanak@google.com, dirk.behme@de.bosch.com, j@jannau.net, fabien.parent@linaro.org, chrisi.schrefl@gmail.com, paulmck@kernel.org, rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, linux-pci@vger.kernel.org, devicetree@vger.kernel.org, rcu@vger.kernel.org, Wedson Almeida Filho Subject: Re: [PATCH v6 06/16] rust: add `Revocable` type Message-ID: References: <20241212163357.35934-7-dakr@kernel.org> <20241218122020.282671-1-benoit@dugarreau.fr> Precedence: bulk X-Mailing-List: rust-for-linux@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20241218122020.282671-1-benoit@dugarreau.fr> On Wed, Dec 18, 2024 at 01:20:20PM +0100, Benoît du Garreau wrote: > On Thu, 12 Dec 2024 17:33:37 +0100 Danilo Krummrich wrote: > > > From: Wedson Almeida Filho > > > > Revocable allows access to objects to be safely revoked at run time. > > > > This is useful, for example, for resources allocated during device probe; > > when the device is removed, the driver should stop accessing the device > > resources even if another state is kept in memory due to existing > > references (i.e., device context data is ref-counted and has a non-zero > > refcount after removal of the device). > > > > Signed-off-by: Wedson Almeida Filho > > Co-developed-by: Danilo Krummrich > > Signed-off-by: Danilo Krummrich > > --- > > rust/kernel/lib.rs | 1 + > > rust/kernel/revocable.rs | 223 +++++++++++++++++++++++++++++++++++++++ > > 2 files changed, 224 insertions(+) > > create mode 100644 rust/kernel/revocable.rs > > > > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs > > index 66149ac5c0c9..5702ce32ec8e 100644 > > --- a/rust/kernel/lib.rs > > +++ b/rust/kernel/lib.rs > > @@ -60,6 +60,7 @@ > > pub mod prelude; > > pub mod print; > > pub mod rbtree; > > +pub mod revocable; > > pub mod security; > > pub mod seq_file; > > pub mod sizes; > > diff --git a/rust/kernel/revocable.rs b/rust/kernel/revocable.rs > > new file mode 100644 > > index 000000000000..e464d59eb6b5 > > --- /dev/null > > +++ b/rust/kernel/revocable.rs > > @@ -0,0 +1,223 @@ > > +// SPDX-License-Identifier: GPL-2.0 > > + > > +//! Revocable objects. > > +//! > > +//! The [`Revocable`] type wraps other types and allows access to them to be revoked. The existence > > +//! of a [`RevocableGuard`] ensures that objects remain valid. > > + > > +use crate::{bindings, prelude::*, sync::rcu, types::Opaque}; > > +use core::{ > > + marker::PhantomData, > > + ops::Deref, > > + ptr::drop_in_place, > > + sync::atomic::{AtomicBool, Ordering}, > > +}; > > + > > +/// An object that can become inaccessible at runtime. > > +/// > > +/// Once access is revoked and all concurrent users complete (i.e., all existing instances of > > +/// [`RevocableGuard`] are dropped), the wrapped object is also dropped. > > +/// > > +/// # Examples > > +/// > > +/// ``` > > +/// # use kernel::revocable::Revocable; > > +/// > > +/// struct Example { > > +/// a: u32, > > +/// b: u32, > > +/// } > > +/// > > +/// fn add_two(v: &Revocable) -> Option { > > +/// let guard = v.try_access()?; > > +/// Some(guard.a + guard.b) > > +/// } > > +/// > > +/// let v = KBox::pin_init(Revocable::new(Example { a: 10, b: 20 }), GFP_KERNEL).unwrap(); > > +/// assert_eq!(add_two(&v), Some(30)); > > +/// v.revoke(); > > +/// assert_eq!(add_two(&v), None); > > +/// ``` > > +/// > > +/// Sample example as above, but explicitly using the rcu read side lock. > > +/// > > +/// ``` > > +/// # use kernel::revocable::Revocable; > > +/// use kernel::sync::rcu; > > +/// > > +/// struct Example { > > +/// a: u32, > > +/// b: u32, > > +/// } > > +/// > > +/// fn add_two(v: &Revocable) -> Option { > > +/// let guard = rcu::read_lock(); > > +/// let e = v.try_access_with_guard(&guard)?; > > +/// Some(e.a + e.b) > > +/// } > > +/// > > +/// let v = KBox::pin_init(Revocable::new(Example { a: 10, b: 20 }), GFP_KERNEL).unwrap(); > > +/// assert_eq!(add_two(&v), Some(30)); > > +/// v.revoke(); > > +/// assert_eq!(add_two(&v), None); > > +/// ``` > > +#[pin_data(PinnedDrop)] > > +pub struct Revocable { > > + is_available: AtomicBool, > > + #[pin] > > + data: Opaque, > > +} > > + > > +// SAFETY: `Revocable` is `Send` if the wrapped object is also `Send`. This is because while the > > +// functionality exposed by `Revocable` can be accessed from any thread/CPU, it is possible that > > +// this isn't supported by the wrapped object. > > +unsafe impl Send for Revocable {} > > + > > +// SAFETY: `Revocable` is `Sync` if the wrapped object is both `Send` and `Sync`. We require `Send` > > +// from the wrapped object as well because of `Revocable::revoke`, which can trigger the `Drop` > > +// implementation of the wrapped object from an arbitrary thread. > > +unsafe impl Sync for Revocable {} > > + > > +impl Revocable { > > + /// Creates a new revocable instance of the given data. > > + pub fn new(data: impl PinInit) -> impl PinInit { > > + pin_init!(Self { > > + is_available: AtomicBool::new(true), > > + data <- Opaque::pin_init(data), > > + }) > > + } > > + > > + /// Tries to access the revocable wrapped object. > > + /// > > + /// Returns `None` if the object has been revoked and is therefore no longer accessible. > > + /// > > + /// Returns a guard that gives access to the object otherwise; the object is guaranteed to > > + /// remain accessible while the guard is alive. In such cases, callers are not allowed to sleep > > + /// because another CPU may be waiting to complete the revocation of this object. > > + pub fn try_access(&self) -> Option> { > > + let guard = rcu::read_lock(); > > + if self.is_available.load(Ordering::Relaxed) { > > + // Since `self.is_available` is true, data is initialised and has to remain valid > > + // because the RCU read side lock prevents it from being dropped. > > + Some(RevocableGuard::new(self.data.get(), guard)) > > + } else { > > + None > > + } > > + } > > + > > + /// Tries to access the revocable wrapped object. > > + /// > > + /// Returns `None` if the object has been revoked and is therefore no longer accessible. > > + /// > > + /// Returns a shared reference to the object otherwise; the object is guaranteed to > > + /// remain accessible while the rcu read side guard is alive. In such cases, callers are not > > + /// allowed to sleep because another CPU may be waiting to complete the revocation of this > > + /// object. > > + pub fn try_access_with_guard<'a>(&'a self, _guard: &'a rcu::Guard) -> Option<&'a T> { > > + if self.is_available.load(Ordering::Relaxed) { > > + // SAFETY: Since `self.is_available` is true, data is initialised and has to remain > > + // valid because the RCU read side lock prevents it from being dropped. > > + Some(unsafe { &*self.data.get() }) > > + } else { > > + None > > + } > > + } > > + > > + /// # Safety > > + /// > > + /// Callers must ensure that there are no more concurrent users of the revocable object. > > + unsafe fn revoke_internal(&self) { > > + if self > > + .is_available > > + .compare_exchange(true, false, Ordering::Relaxed, Ordering::Relaxed) > > + .is_ok() > > + { > > The comment I made in a previous series was somehow lost, so I put it back here: > You can use `self.is_available.swap(false, Ordering::Relaxed)` instead of a CAS, > it is IMO clearer and optimizes better on some architectures. Thanks for bringing this up again! > > > + if SYNC { > > + // SAFETY: Just an FFI call, there are no further requirements. > > + unsafe { bindings::synchronize_rcu() }; > > + } > > + > > + // SAFETY: We know `self.data` is valid because only one CPU can succeed the > > + // `compare_exchange` above that takes `is_available` from `true` to `false`. > > + unsafe { drop_in_place(self.data.get()) }; > > + } > > + } > > + > > + /// Revokes access to and drops the wrapped object. > > + /// > > + /// Access to the object is revoked immediately to new callers of [`Revocable::try_access`], > > + /// expecting that there are no concurrent users of the object. > > + /// > > + /// # Safety > > + /// > > + /// Callers must ensure that there are no more concurrent users of the revocable object. > > + pub unsafe fn revoke_nosync(&self) { > > + // SAFETY: By the safety requirement of this function, the caller ensures that nobody is > > + // accessing the data anymore and hence we don't have to wait for the grace period to > > + // finish. > > + unsafe { self.revoke_internal::() } > > + } > > + > > + /// Revokes access to and drops the wrapped object. > > + /// > > + /// Access to the object is revoked immediately to new callers of [`Revocable::try_access`]. > > + /// > > + /// If there are concurrent users of the object (i.e., ones that called > > + /// [`Revocable::try_access`] beforehand and still haven't dropped the returned guard), this > > + /// function waits for the concurrent access to complete before dropping the wrapped object. > > + pub fn revoke(&self) { > > + // SAFETY: By passing `true` we ask `revoke_internal` to wait for the grace period to > > + // finish. > > + unsafe { self.revoke_internal::() } > > + } > > +} > > + > > +#[pinned_drop] > > +impl PinnedDrop for Revocable { > > + fn drop(self: Pin<&mut Self>) { > > + // Drop only if the data hasn't been revoked yet (in which case it has already been > > + // dropped). > > + // SAFETY: We are not moving out of `p`, only dropping in place > > + let p = unsafe { self.get_unchecked_mut() }; > > + if *p.is_available.get_mut() { > > + // SAFETY: We know `self.data` is valid because no other CPU has changed > > + // `is_available` to `false` yet, and no other CPU can do it anymore because this CPU > > + // holds the only reference (mutable) to `self` now. > > + unsafe { drop_in_place(p.data.get()) }; > > + } > > + } > > +} > > + > > +/// A guard that allows access to a revocable object and keeps it alive. > > +/// > > +/// CPUs may not sleep while holding on to [`RevocableGuard`] because it's in atomic context > > +/// holding the RCU read-side lock. > > +/// > > +/// # Invariants > > +/// > > +/// The RCU read-side lock is held while the guard is alive. > > +pub struct RevocableGuard<'a, T> { > > + data_ref: *const T, > > + _rcu_guard: rcu::Guard, > > + _p: PhantomData<&'a ()>, > > +} > > + > > +impl RevocableGuard<'_, T> { > > + fn new(data_ref: *const T, rcu_guard: rcu::Guard) -> Self { > > + Self { > > + data_ref, > > + _rcu_guard: rcu_guard, > > + _p: PhantomData, > > + } > > + } > > +} > > + > > +impl Deref for RevocableGuard<'_, T> { > > + type Target = T; > > + > > + fn deref(&self) -> &Self::Target { > > + // SAFETY: By the type invariants, we hold the rcu read-side lock, so the object is > > + // guaranteed to remain valid. > > + unsafe { &*self.data_ref } > > + } > > +} > > -- > > 2.47.1 > > > > > > Benoît du Garreau