[PATCH v19 5/5] rust: sync: Introduce SpinLockIrq::lock_with() and friends

Lyude Paul
2026-03-02T23:22:26Z

`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 InterruptDisabled) -> Guard<'a, .., SpinLockBackend>

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 <lyude@redhat.com>
Co-developed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Gary Guo <gary@garyguo.net>

---
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<T, B> to Lock<T, B::ContextualBackend> 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 = …] 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()...
V18:
* Get rid of BackendWithContext
* Just use transmute in as_lock_in_context()
* Now that we're only supporting IRQ spinlocks and not using traits, use
the type aliases for SpinLock and SpinLockGuard
* Improve the docs now that we're not using traits.
V19:
* #[inline] #[inline] #[inline]

rust/kernel/sync/lock/spinlock.rs | 81 ++++++++++++++++++++++++++++++-
1 file changed, 80 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/sync/lock/spinlock.rs b/rust/kernel/sync/lock/spinlock.rs
index 6b8f92d5b0467..069fcdb58735a 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.
///
@@ -224,6 +227,45 @@ 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<Inner>,
+/// }
+///
+/// impl Example {
+/// fn new() -> impl PinInit<Self> {
+/// 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<T> = super::Lock<T, SpinLockIrqBackend>;
@@ -291,6 +333,43 @@ unsafe fn assert_is_held(ptr: *mut Self::State) {
}
}

+impl<T: ?Sized> Lock<T, SpinLockIrqBackend> {
+ /// Casts the lock as a `Lock<T, SpinLockBackend>`.
+ #[inline]
+ fn as_lock_in_interrupt<'a>(&'a self, _context: &'a LocalInterruptDisabled) -> &'a SpinLock<T> {
+ // SAFETY:
+ // - `Lock<T, SpinLockBackend>` and `Lock<T, SpinLockIrqBackend>` 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<SpinLockGuard<'a, T>> {
+ self.as_lock_in_interrupt(context).try_lock()
+ }
+}
+
#[kunit_tests(rust_spinlock_irq_condvar)]
mod tests {
use super::*;
--
2.53.0