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 2EED647728A for ; Tue, 4 Aug 2026 16:15:27 +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=1785860136; cv=none; b=E3bdDOuONTfd6Rt9ydi7Y5BPU3tI9+1pem6cqVzDrtK2RLERGtFKsMPvhh5ncqDzO3+9hz4dSaWfdaC3iOsiURbbs9vpT0D/r8MadkdQcNRn0UUwEaCewSF+O9wZjS1tdN0N1ZwlXGWAzXGNBQ26Bdv4df1Il8qC7eCquDqz/8U= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785860136; c=relaxed/simple; bh=yOW+vEyMmlLd/rvdAQOpfI7rRLssPojgy8OC8Xdb7b8=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=cwh3Cb/zAJKoLsF5Wz6JXw3VtP4FuOAk/+Oed9XXUurnrs3wwGZCQw4G6CSWQRj4qPbqFUkaOHWPhyR9kNb0yiBRwhzR4Nez2bv/X/WTLPIKYOEz+LvWCF4NooQ94d2ocFwL1wWxa1d3ybSAyzJRmcldMvS7ccYC5SmuaQlnO4w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=GhtIyMs+; 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="GhtIyMs+" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A29011F00ADE; Tue, 4 Aug 2026 16:15:24 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1785860124; bh=J5r9F5NVhPBA6Gj8qaTBPl3yuPTDh/upo+Mtrw3vN6s=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=GhtIyMs+lDB/3WxFofpCVhr5lrAJAfiK/2Jmz7Yb5ccySl6rQotVGNNSWbwDzH6Om 5Gcqpn8c9dn9yEjLDupABxeqD8aVZe+jNHQt87w51hpe8Aisu1n5gTEB6hUN1sr04K Gny5kXFM855sCVusdcQx1PtuOiBbXj8qOONk6gmFMWgll6rYu9ErqsC+PaKfP1Lm6M Jh1Azipm7vIi1qGfu7hC+ZpJ+Szgs3w82j/m63cJlIG/rS2gm2Xp0JWO7BIoMO5oT8 GudAFFM60dImjdkX3Ev5dYcZgnd9t96v4w5AE2gcX8M1Mf0udewI2WxnGcHSnz3tCB yY/hvJOLy2TiQ== Received: from phl-compute-05.internal (phl-compute-05.internal [10.202.2.45]) by mailfauth.phl.internal (Postfix) with ESMTP id E0698F40068; Tue, 4 Aug 2026 12:15:23 -0400 (EDT) Received: from phl-frontend-03 ([10.202.2.162]) by phl-compute-05.internal (MEProxy); Tue, 04 Aug 2026 12:15:23 -0400 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: dmFkZTE6f1b/PLD7WfUEjv3UPH0IuooDzdONFOoQTajOMPpDZsgOHZQOVqJ4v+RHATO1sj EZSfNibFX853su8vPqAqnUSuvHiUbplQY0egsm0mTGT1lvu1S/5Nh/tuqaumzTAbEEqn88 A4HuRrhsSYsTxma98RYyzM2Gbo4E2wg96qbDSuNUia/kyZtd9PuUf9ikEEX+FWBaYhk2LZ 4A6T9U94iL3j6tNfmRpUMp/Z8p6+0GWHZbXf2pAo98Kg1ESY7btybSSSUnrcHypkA1CoVm T+WboQPfBLNhjm5E0DtWi31bTX/atiaNKg8ICM8aktklu407EF+Nx0w3ySFDe9rX99/5c7 VHxd0IBeRLjSglLmGGu4qcFo6l3dFM8dlqsmahN6miVS0lDN1uZWPAhgUWk58n5dNfTejY O3XlDdY3zr/a45OAr6aLsOyn9N9frGmsOzWcXGIDsrsKwxBBbBdBSHbc7U+/PXIzYEdnIp XLi0Fm5TYkwEKFgsBYim6ZlA6G3VRnEbfq+bx4qUJtD9jjD+UHYqCmFNGzXeZZUwbGHCjb 0FrfHhihkhv4i0JVaWFshEQPJ8wWTnJhu59no68G7UkSd/B0+bGTmdVG/7sYc/sGzR0Guq kHc5SXgxoWcLPZfgI2Ldw1o/s1T7ccCNIeshuUsOQ+f6gYWEB4R/m+m9jv2g X-ME-Proxy: Feedback-ID: i8dbe485b:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Tue, 4 Aug 2026 12:15:23 -0400 (EDT) From: Boqun Feng To: Peter Zijlstra Cc: "Ingo Molnar" , "Will Deacon" , "Boqun Feng" , "Waiman Long" , "Gary Guo" , "Alice Ryhl" , "Lyude Paul" , "Daniel Almeida" , =?UTF-8?q?Onur=20=C3=96zkan?= , "Miguel Ojeda" , "Danilo Krummrich" , linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org Subject: [PATCH v4 17/17] rust: sync: Introduce SpinLockIrq::lock_with() and friends Date: Tue, 4 Aug 2026 09:14:39 -0700 Message-ID: <20260804161447.84806-18-boqun@kernel.org> X-Mailer: git-send-email 2.50.1 In-Reply-To: <20260804161447.84806-1-boqun@kernel.org> References: <20260804161447.84806-1-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 From: Lyude Paul `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. In order to do this, we add some special functions for SpinLockIrq: lock_with() and try_lock_with(), which allow acquiring the lock without changing the interrupt state - as long as the caller can provide a LocalInterruptDisabled reference to prove that local processor interrupts have been disabled. 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. This makes it so that a `SpinLockIrq` will work like a `SpinLock` if interrupts are disabled. So a function: (&'a SpinLockIrq, &'a LocalInterruptDisabled) -> Guard<'a, .., SpinLockBackend> makes sense. Note that due to `Guard` and `LocalInterruptDisabled` having the same lifetime, interrupts cannot be enabled while the Guard exists. Signed-off-by: Lyude Paul Reviewed-by: Gary Guo Signed-off-by: Boqun Feng --- rust/kernel/sync/lock/spinlock.rs | 92 ++++++++++++++++++++++++++++++- 1 file changed, 91 insertions(+), 1 deletion(-) diff --git a/rust/kernel/sync/lock/spinlock.rs b/rust/kernel/sync/lock/spinlock.rs index 872544948e5d..aafc80125f59 100644 --- a/rust/kernel/sync/lock/spinlock.rs +++ b/rust/kernel/sync/lock/spinlock.rs @@ -4,7 +4,10 @@ //! //! This module allows Rust code to use the kernel's `spinlock_t`. use super::*; -use crate::prelude::*; +use crate::{ + interrupt::LocalInterruptDisabled, + prelude::*, // +}; /// Creates a [`SpinLock`] initialiser with the given name and a newly-created lock class. /// @@ -160,6 +163,16 @@ macro_rules! new_spinlock_irq { /// A variant of `SpinLock` that ensures interrupts are disabled in the critical section. /// +/// This lock can be acquired in two ways: +/// +/// - Using [`lock()`] like any other type of lock, in which case the bindings will modify the +/// interrupt state to ensure that local processor interrupts remain disabled for at least as +/// long as the [`SpinLockIrqGuard`] exists. +/// - Using [`lock_with()`] in contexts where a [`LocalInterruptDisabled`] token is present and +/// local processor interrupts are already known to be disabled, in which case the local +/// interrupt state will not be touched. This method should be preferred if a +/// [`LocalInterruptDisabled`] token is present in the scope. +/// /// For more info on spinlocks, see [`SpinLock`]. For more information on interrupts, /// [see the interrupt module](kernel::interrupt). /// @@ -213,7 +226,47 @@ macro_rules! new_spinlock_irq { /// # Ok::<(), Error>(()) /// ``` /// +/// The next example demonstrates locking a [`SpinLockIrq`] using [`lock_with()`] in a function +/// which can only be called when local processor interrupts are already disabled. +/// +/// ``` +/// 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 no-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 = KBox::pin_init(Example::new(), GFP_KERNEL)?; +/// # let interrupt_guard = local_interrupt_disable(); +/// # noirq_work(&e, &interrupt_guard); +/// # +/// # Ok::<(), Error>(()) +/// ``` +/// /// [`lock()`]: SpinLockIrq::lock +/// [`lock_with()`]: SpinLockIrq::lock_with pub type SpinLockIrq = super::Lock; /// A kernel `spinlock_t` lock backend that can only be acquired in interrupt disabled contexts. @@ -278,6 +331,43 @@ unsafe fn assert_is_held(ptr: *mut Self::State) { } } +impl Lock { + /// Casts the lock as a `Lock`. + #[inline] + fn as_lock_in_interrupt<'a>(&'a self, _context: &'a LocalInterruptDisabled) -> &'a SpinLock { + // SAFETY: + // - `Lock` and `Lock` both have identical data + // layouts. + // - As long as local interrupts are disabled (which is proven to be true by _context), it + // is safe to treat a lock with SpinLockIrqBackend as a SpinLockBackend lock. + unsafe { core::mem::transmute(self) } + } + + /// Acquires the lock without modifying local interrupt state. + /// + /// This function should be used in place of the more expensive [`Lock::lock()`] function when + /// possible for [`SpinLockIrq`] locks. + #[inline] + pub fn lock_with<'a>(&'a self, context: &'a LocalInterruptDisabled) -> SpinLockGuard<'a, T> { + self.as_lock_in_interrupt(context).lock() + } + + /// Tries to acquire the lock without modifying local interrupt state. + /// + /// This function should be used in place of the more expensive [`Lock::try_lock()`] function + /// when possible for [`SpinLockIrq`] locks. + /// + /// Returns a guard that can be used to access the data protected by the lock if successful. + #[must_use = "if unused, the lock will be immediately unlocked"] + #[inline] + pub fn try_lock_with<'a>( + &'a self, + context: &'a LocalInterruptDisabled, + ) -> Option> { + self.as_lock_in_interrupt(context).try_lock() + } +} + #[kunit_tests(rust_spinlock_irq_condvar)] mod tests { use super::*; -- 2.50.1 (Apple Git-155)