From nobody Sat Feb 7 08:02:25 2026 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 B397D44BCB7 for ; Wed, 21 Jan 2026 22:40:33 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769035240; cv=none; b=jt0t3wnfIHbPfdyjq7KrVsMChEqjGGjqodCGcjhErk4z0+9GeUphbLpznmIkYxdEEXaIASDvCrJrc9R9tXmxxSaN/h2PwiBDrCC/ZwtphSUBiUqZIinMUKxioZ+jjyb0QzCrhgu05T7TC0dxHU4i4efw8s9ZCNbyI+5gOL8GrHg= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1769035240; c=relaxed/simple; bh=Y5i+GVPEllObdijfuWZ9I1W6YV+9xv+1duARbR47X9A=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=ELcHpYYWFvWE0m5JP3ILc0wZ6Oec84kdWZhId+l4oVXoybldtDwTzihEYHJwmsw/AKaPv6dAuDkKoq+sHOyb5lEVuJefKVjMmQNe78qQ3C8rHDijd/3uMubloHWrYdCf5jQNvNwHDDQGGu5EV04Y9mPWAqFEFXiRvzThk2ABaQ4= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=jFdqG+T+; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="jFdqG+T+" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1769035229; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Bb3vM/HltnILUnQCfknRXYpdwRws5q5vWcnYkLy8n18=; b=jFdqG+T+/kxzURnf1sfeWV81r/2cfRDExHC86DCSEqIpe8PPieusPKYTePn9j6npYsSAuH 5ztsOBQtJFwLZTFFBHDHgLWfF4rEQ9FYrlfqzYCOFEfu2blp0ZCABSWdxfTo8KqvSTGJ2m RtqIXZ9Ku8BT7q5d+uYuY6ZRV3n0xkU= Received: from mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-484-mctYfoVCPhqxLArjR_Gecg-1; Wed, 21 Jan 2026 17:40:25 -0500 X-MC-Unique: mctYfoVCPhqxLArjR_Gecg-1 X-Mimecast-MFC-AGG-ID: mctYfoVCPhqxLArjR_Gecg_1769035223 Received: from mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.17]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 3E62C1956080; Wed, 21 Jan 2026 22:40:23 +0000 (UTC) Received: from GoldenWind.redhat.com (unknown [10.22.89.232]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 9F72019560AB; Wed, 21 Jan 2026 22:40:20 +0000 (UTC) From: Lyude Paul To: rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, Thomas Gleixner Cc: Boqun Feng , Daniel Almeida , Miguel Ojeda , Alex Gaynor , Gary Guo , =?UTF-8?q?Bj=C3=B6rn=20Roy=20Baron?= , Benno Lossin , Andreas Hindborg , Alice Ryhl , Trevor Gross , Danilo Krummrich , Andrew Morton , Peter Zijlstra , Ingo Molnar , Will Deacon , Waiman Long Subject: [PATCH v17 10/16] rust: sync: Introduce lock::Lock::lock_with() and friends Date: Wed, 21 Jan 2026 17:39:13 -0500 Message-ID: <20260121223933.1568682-11-lyude@redhat.com> In-Reply-To: <20260121223933.1568682-1-lyude@redhat.com> References: <20260121223933.1568682-1-lyude@redhat.com> 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="utf-8" Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 `SpinLockIrq` and `SpinLock` use the exact same underlying C structure, with the only real difference being that the former uses the irq_disable() and irq_enable() variants for locking/unlocking. These variants can introduce some minor overhead in contexts where we already know that local processor interrupts are disabled, and as such we want a way to be able to skip modifying processor interrupt state in said contexts in order to avoid some overhead - just like the current C API allows us to do. So, `ContextualBackend` allows us to cast a lock into it's contextless version for situations where we already have whatever guarantees would be provided by `BackendWithContext::ContextualBackend` in place. In some hacked-together benchmarks we ran, most of the time this did actually seem to lead to a noticeable difference in overhead: From an aarch64 VM running on a MacBook M4: lock() when irq is disabled, 100 times cost Delta { nanos: 500 } lock_with() when irq is disabled, 100 times cost Delta { nanos: 292 } lock() when irq is enabled, 100 times cost Delta { nanos: 834 } lock() when irq is disabled, 100 times cost Delta { nanos: 459 } lock_with() when irq is disabled, 100 times cost Delta { nanos: 291 } lock() when irq is enabled, 100 times cost Delta { nanos: 709 } From an x86_64 VM (qemu/kvm) running on a i7-13700H lock() when irq is disabled, 100 times cost Delta { nanos: 1002 } lock_with() when irq is disabled, 100 times cost Delta { nanos: 729 } lock() when irq is enabled, 100 times cost Delta { nanos: 1516 } lock() when irq is disabled, 100 times cost Delta { nanos: 754 } lock_with() when irq is disabled, 100 times cost Delta { nanos: 966 } lock() when irq is enabled, 100 times cost Delta { nanos: 1227 } (note that there were some runs on x86_64 where lock() on irq disabled vs. lock_with() on irq disabled had equivalent benchmarks, but it very much appeared to be a minority of test runs. While it's not clear how this affects real-world workloads yet, let's add this for the time being so we can find out. Implement lock::Lock::lock_with() and lock::BackendWithContext::ContextualBackend. This makes it so that a `SpinLockIrq` will work like a `SpinLock` if interrupts are disabled. So a function: (&'a SpinLockIrq, &'a InterruptDisabled) -> Guard<'a, .., SpinLockB= ackend> makes sense. Note that due to `Guard` and `InterruptDisabled` having the same lifetime, interrupts cannot be enabled while the Guard exists. Signed-off-by: Lyude Paul Co-developed-by: Boqun Feng Signed-off-by: Boqun Feng --- This was originally two patches, but keeping them split didn't make sense after going from BackendInContext to BackendWithContext. V10: * Fix typos - Dirk/Lyude * Since we're adding support for context locks to GlobalLock as well, let's also make sure to cover try_lock while we're at it and add try_lock_with * Add a private function as_lock_in_context() for handling casting from a Lock to Lock so we don't have to duplicate safety comments V11: * Fix clippy::ref_as_ptr error in Lock::as_lock_in_context() V14: * Add benchmark results, rewrite commit message V17: * Introduce `BackendWithContext`, move context-related bits into there and out of `Backend`. * Add missing #[must_use =3D =E2=80=A6] for try_lock_with() * Remove all unsafe code from lock_with() and try_lock_with(): Somehow I never noticed that literally none of the unsafe code in these two functions is needed with as_lock_in_context()... rust/kernel/sync/lock.rs | 71 ++++++++++++++++++++++++++++++- rust/kernel/sync/lock/spinlock.rs | 48 ++++++++++++++++++++- 2 files changed, 117 insertions(+), 2 deletions(-) diff --git a/rust/kernel/sync/lock.rs b/rust/kernel/sync/lock.rs index 46a57d1fc309d..9f6d7b381bd15 100644 --- a/rust/kernel/sync/lock.rs +++ b/rust/kernel/sync/lock.rs @@ -30,10 +30,15 @@ /// is owned, that is, between calls to [`lock`] and [`unlock`]. /// - Implementers must also ensure that [`relock`] uses the same locking = method as the original /// lock operation. +/// - Implementers must ensure if [`BackendInContext`] is a [`Backend`], i= t's safe to acquire the +/// lock under the [`Context`], the [`State`] of two backends must be th= e same. /// /// [`lock`]: Backend::lock /// [`unlock`]: Backend::unlock /// [`relock`]: Backend::relock +/// [`BackendInContext`]: Backend::BackendInContext +/// [`Context`]: Backend::Context +/// [`State`]: Backend::State pub unsafe trait Backend { /// The state required by the lock. type State; @@ -97,6 +102,34 @@ unsafe fn relock(ptr: *mut Self::State, guard_state: &m= ut Self::GuardState) { unsafe fn assert_is_held(ptr: *mut Self::State); } =20 +/// A lock [`Backend`] with a [`ContextualBackend`] that can make lock acq= uisition cheaper. +/// +/// Some locks, such as [`SpinLockIrq`](super::SpinLockIrq), can only be a= cquired in specific +/// hardware contexts (e.g. local processor interrupts disabled). Entering= and exiting these +/// contexts incurs additional overhead. But this overhead may be avoided = if we know ahead of time +/// that we are already within the correct context for a given lock as we = can then skip any costly +/// operations required for entering/exiting said context. +/// +/// Any lock implementing this trait requires such a interrupt context, an= d can provide cheaper +/// lock-acquisition functions through [`Lock::lock_with`] and [`Lock::try= _lock_with`] as long as a +/// context token of type [`Context`] is available. +/// +/// # Safety +/// +/// - Implementors must ensure that it is safe to acquire the lock under [= `Context`]. +/// +/// [`ContextualBackend`]: BackendWithContext::ContextualBackend +/// [`Context`]: BackendWithContext::Context +pub unsafe trait BackendWithContext: Backend { + /// The context which must be provided in order to acquire the lock wi= th the + /// [`ContextualBackend`](BackendWithContext::ContextualBackend). + type Context<'a>; + + /// The alternative cheaper backend we can use if a [`Context`](Backen= dWithContext::Context) is + /// provided. + type ContextualBackend: Backend; +} + /// A mutual exclusion primitive. /// /// Exposes one of the kernel locking primitives. Which one is exposed dep= ends on the lock @@ -169,7 +202,8 @@ pub unsafe fn from_raw<'a>(ptr: *mut B::State) -> &'a S= elf { =20 impl Lock { /// Acquires the lock and gives the caller access to the data protecte= d by it. - pub fn lock(&self) -> Guard<'_, T, B> { + #[inline] + pub fn lock<'a>(&'a self) -> Guard<'a, T, B> { // SAFETY: The constructor of the type calls `init`, so the existe= nce of the object proves // that `init` was called. let state =3D unsafe { B::lock(self.state.get()) }; @@ -189,6 +223,41 @@ pub fn try_lock(&self) -> Option> { } } =20 +impl Lock { + /// Casts the lock as a `Lock`. + fn as_lock_in_context<'a>( + &'a self, + _context: B::Context<'a>, + ) -> &'a Lock + where + B::ContextualBackend: Backend, + { + // SAFETY: + // - Per the safety guarantee of `Backend`, if `B::ContextualBacke= nd` and `B` should + // have the same state, the layout of the lock is the same so it= 's safe to convert one to + // another. + // - The caller provided `B::Context<'a>`, so it is safe to recast= and return this lock. + unsafe { &*(core::ptr::from_ref(self) as *const _) } + } + + /// Acquires the lock with the given context and gives the caller acce= ss to the data protected + /// by it. + pub fn lock_with<'a>(&'a self, context: B::Context<'a>) -> Guard<'a, T= , B::ContextualBackend> { + self.as_lock_in_context(context).lock() + } + + /// Tries to acquire the lock with the given context. + /// + /// Returns a guard that can be used to access the data protected by t= he lock if successful. + #[must_use =3D "if unused, the lock will be immediately unlocked"] + pub fn try_lock_with<'a>( + &'a self, + context: B::Context<'a>, + ) -> Option> { + self.as_lock_in_context(context).try_lock() + } +} + /// A lock guard. /// /// Allows mutual exclusion primitives that implement the [`Backend`] trai= t to automatically unlock diff --git a/rust/kernel/sync/lock/spinlock.rs b/rust/kernel/sync/lock/spin= lock.rs index 3fdfb0a8a0ab1..e082791a0d23c 100644 --- a/rust/kernel/sync/lock/spinlock.rs +++ b/rust/kernel/sync/lock/spinlock.rs @@ -3,7 +3,7 @@ //! A kernel spinlock. //! //! This module allows Rust code to use the kernel's `spinlock_t`. -use crate::prelude::*; +use crate::{interrupt::LocalInterruptDisabled, prelude::*}; =20 /// Creates a [`SpinLock`] initialiser with the given name and a newly-cre= ated lock class. /// @@ -220,6 +220,45 @@ macro_rules! new_spinlock_irq { /// # Ok::<(), Error>(()) /// ``` /// +/// The next example demonstrates locking a [`SpinLockIrq`] using [`lock_w= ith()`] in a function +/// which can only be called when local processor interrupts are already d= isabled. +/// +/// ``` +/// use kernel::sync::{new_spinlock_irq, SpinLockIrq}; +/// use kernel::interrupt::*; +/// +/// struct Inner { +/// a: u32, +/// } +/// +/// #[pin_data] +/// struct Example { +/// #[pin] +/// inner: SpinLockIrq, +/// } +/// +/// impl Example { +/// fn new() -> impl PinInit { +/// pin_init!(Self { +/// inner <- new_spinlock_irq!(Inner { a: 20 }), +/// }) +/// } +/// } +/// +/// // Accessing an `Example` from a function that can only be called in n= o-interrupt contexts. +/// fn noirq_work(e: &Example, interrupt_disabled: &LocalInterruptDisabled= ) { +/// // Because we know interrupts are disabled from interrupt_disable,= we can skip toggling +/// // interrupt state using lock_with() and the provided token +/// assert_eq!(e.inner.lock_with(interrupt_disabled).a, 20); +/// } +/// +/// # let e =3D KBox::pin_init(Example::new(), GFP_KERNEL)?; +/// # let interrupt_guard =3D local_interrupt_disable(); +/// # noirq_work(&e, &interrupt_guard); +/// # +/// # Ok::<(), Error>(()) +/// ``` +/// /// [`lock()`]: SpinLockIrq::lock /// [`lock_with()`]: SpinLockIrq::lock_with pub type SpinLockIrq =3D super::Lock; @@ -283,6 +322,13 @@ unsafe fn assert_is_held(ptr: *mut Self::State) { } } =20 +// SAFETY: When executing with local processor interrupts disabled, [`Spin= Lock`] and [`SpinLockIrq`] +// are identical. +unsafe impl super::BackendWithContext for SpinLockIrqBackend { + type Context<'a> =3D &'a LocalInterruptDisabled; + type ContextualBackend =3D SpinLockBackend; +} + #[kunit_tests(rust_spinlock_irq_condvar)] mod tests { use super::*; --=20 2.52.0