Message-Id: <DG7EYT29W3CY.2WMEH783JP7RI@garyguo.net>
Date: Thu, 05 Feb 2026 23:23:47 +0000
From: "Gary Guo" <gary@...yguo.net>
To: "Lyude Paul" <lyude@...hat.com>, <rust-for-linux@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, "Thomas Gleixner" <tglx@...utronix.de>
Cc: "Boqun Feng" <boqun.feng@...il.com>, "Daniel Almeida"
 <daniel.almeida@...labora.com>, "Miguel Ojeda" <ojeda@...nel.org>, "Alex
 Gaynor" <alex.gaynor@...il.com>, "Gary Guo" <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Benno Lossin"
 <lossin@...nel.org>, "Andreas Hindborg" <a.hindborg@...nel.org>, "Alice
 Ryhl" <aliceryhl@...gle.com>, "Trevor Gross" <tmgross@...ch.edu>, "Danilo
 Krummrich" <dakr@...nel.org>, "Andrew Morton" <akpm@...ux-foundation.org>,
 "Peter Zijlstra" <peterz@...radead.org>, "Ingo Molnar" <mingo@...hat.com>,
 "Will Deacon" <will@...nel.org>, "Waiman Long" <longman@...hat.com>
Subject: Re: [PATCH v18 5/5] rust: sync: Introduce SpinLockIrq::lock_with()
 and friends

On Thu Feb 5, 2026 at 8:44 PM GMT, Lyude Paul wrote:
> `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@...hat.com>
> Co-developed-by: Boqun Feng <boqun.feng@...il.com>
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
>
> ---
> 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.
>
>  rust/kernel/sync/lock/spinlock.rs | 78 ++++++++++++++++++++++++++++++-
>  1 file changed, 77 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/sync/lock/spinlock.rs b/rust/kernel/sync/lock/spinlock.rs
> index f11a84505ba0e..de736cb777e93 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>;
> @@ -286,6 +328,40 @@ unsafe fn assert_is_held(ptr: *mut Self::State) {
>      }
>  }
>  

This is much cleaner now compared the previous series. Thanks!

With functions marked as `#[inline]` as appropriate:

    Reviewed-by: Gary Guo <gary@...yguo.net>

BTW: do we want to have a

    impl<'a> lock::Guard<'a, T, SpinLockIrqBackend> {
        fn as_local_irq_disabled(&self) -> &LocalInterruptDisabled;
    }

?

Best,
Gary

> +impl<T: ?Sized> Lock<T, SpinLockIrqBackend> {
> +    /// Casts the lock as a `Lock<T, SpinLockBackend>`.
> +    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.
> +    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"]
> +    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::*;

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