Re: [PATCH 1/3] rust: Introduce irq module

[Lyude Paul]

On Fri, 2024-07-26 at 07:23 +0000, Benno Lossin wrote:
> On 26.07.24 00:27, Lyude Paul wrote:
> > This introduces a module for dealing with interrupt-disabled contexts,
> > including the ability to enable and disable interrupts
> > (with_irqs_disabled()) - along with the ability to annotate functions as
> > expecting that IRQs are already disabled on the local CPU.
> > 
> > Signed-off-by: Lyude Paul <lyude@xxxxxxxxxx>
> > ---
> >  rust/helpers.c     | 14 +++++++++
> >  rust/kernel/irq.rs | 74 ++++++++++++++++++++++++++++++++++++++++++++++
> >  rust/kernel/lib.rs |  1 +
> >  3 files changed, 89 insertions(+)
> >  create mode 100644 rust/kernel/irq.rs
> > 
> > diff --git a/rust/helpers.c b/rust/helpers.c
> > index 87ed0a5b60990..12ac32de820b5 100644
> > --- a/rust/helpers.c
> > +++ b/rust/helpers.c
> > @@ -69,6 +69,20 @@ void rust_helper_spin_unlock(spinlock_t *lock)
> >  }
> >  EXPORT_SYMBOL_GPL(rust_helper_spin_unlock);
> > 
> > +unsigned long rust_helper_local_irq_save(void) {
> > +	unsigned long flags;
> > +
> > +	local_irq_save(flags);
> > +
> > +	return flags;
> > +}
> > +EXPORT_SYMBOL_GPL(rust_helper_local_irq_save);
> > +
> > +void rust_helper_local_irq_restore(unsigned long flags) {
> > +	local_irq_restore(flags);
> > +}
> > +EXPORT_SYMBOL_GPL(rust_helper_local_irq_restore);
> > +
> >  void rust_helper_init_wait(struct wait_queue_entry *wq_entry)
> >  {
> >  	init_wait(wq_entry);
> > diff --git a/rust/kernel/irq.rs b/rust/kernel/irq.rs
> > new file mode 100644
> > index 0000000000000..8a540bd6123f7
> > --- /dev/null
> > +++ b/rust/kernel/irq.rs
> > @@ -0,0 +1,74 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +//! Interrupt controls
> > +//!
> > +//! This module allows Rust code to control processor interrupts. [`with_irqs_disabled()`] may be
> > +//! used for nested disables of interrupts, whereas [`IrqDisabled`] can be used for annotating code
> > +//! that requires that interrupts already be disabled.
> > +
> > +use bindings;
> > +use core::marker::*;
> > +
> > +/// A guarantee that IRQs are disabled on this CPU
> > +///
> > +/// An [`IrqDisabled`] represents a guarantee that interrupts will remain disabled on the current CPU
> > +/// until the lifetime of the object ends. However, it does not disable or enable interrupts on its
> > +/// own - see [`with_irqs_disabled()`] for that.
> > +///
> > +/// This object has no cost at runtime (TODO: …except if whatever kernel compile-time option that
> > +/// would assert IRQs are enabled or not is enabled - in which case we should actually verify that
> > +/// they're enabled).
> > +///
> > +/// # Examples
> > +///
> > +/// If you want to ensure that a function may only be invoked within contexts where interrupts are
> > +/// disabled, you can do so by requiring that a reference to this type be passed. You can also
> > +/// create this type using unsafe code in order to indicate that it's known that interrupts are
> > +/// already disabled on this CPU
> > +///
> > +/// ```
> > +/// use kernel::irq::{IrqDisabled, disable_irqs};
> > +///
> > +/// // Requiring interrupts be disabled to call a function
> > +/// fn dont_interrupt_me(_irq: &IrqDisabled<'_>) { }
> 
> I would expect the function to take `IrqDisabled` by value instead of by
> reference.
> 
> > +///
> > +/// // Disabling interrupts. They'll be re-enabled once this closure completes.
> > +/// disable_irqs(|irq| dont_interrupt_me(&irq));
> 
> Because then you don't need a borrow (`&`) here.
> 
> > +/// ```
> > +pub struct IrqDisabled<'a>(PhantomData<&'a ()>);
> 
> You would also need to `#[derive(Clone, Copy)]` and since we're at it, I
> would also add `Debug, Ord, Eq, PartialOrd, PartialEq, Hash`.
> The last ones are important if we want to have structs that can only
> exist while IRQs are disabled. I don't know if that makes sense, but I
> think it's fine to add the derives now.

sgtm

> 
> Another thing, I am wondering if we want this to be invariant over the
> lifetime, I don't have a good reason, but I still think we should
> consider it.
> 
> > +
> > +impl<'a> IrqDisabled<'a> {
> > +    /// Create a new [`IrqDisabled`] without disabling interrupts
> > +    ///
> > +    /// If debug assertions are enabled, this function will check that interrupts are disabled.
> > +    /// Otherwise, it has no cost at runtime.
> 
> I don't see a check in the function below.

Agh, thanks for pointing this out! I totally forgot I wanted to investigate
how to do this before submitting, so I'll look into that today.

> 
> > +    ///
> > +    /// # Safety
> > +    ///
> > +    /// This function must only be called in contexts where it is already known that interrupts have
> > +    /// been disabled for the current CPU, as the user is making a promise that they will remain
> > +    /// disabled at least until this [`IrqDisabled`] is dropped.
> > +    pub unsafe fn new() -> Self {
> > +        Self(PhantomData)
> > +    }
> 
> What about adding a function here (taking `self` or `&self`, it doesn't
> matter if you derived `Copy`) that checks if IRQs are disabled when
> debug assertions are on?

sgtm of course

> 
> > +}
> > +
> > +/// Run the closure `cb` with interrupts disabled on the local CPU.
> > +///
> > +/// Interrupts will be re-enabled once the closure returns. If interrupts were already disabled on
> > +/// this CPU, this is a no-op.
> > +#[inline]
> > +pub fn with_irqs_disabled<T, F>(cb: F) -> T
> > +where
> > +    F: FnOnce(IrqDisabled<'_>) -> T,
> > +{
> > +    // SAFETY: FFI call with no special requirements
> 
> I vaguely remember that there were some problems with sleeping in IRQ
> disabled contexts, is that me just misremembering (eg confusing it with
> atomic context), or do we need to watch out for that?

You're correct - sleeping isn't allowed in no-irq contexts. 

> Because if that is the case, then we would need to use klint.

Ok - I've never used klint before but I'm happy to look into it and try to
implement something for it.

FWIW too: I assume we would still want klint anyway, but I think it's at least
worth mentioning the kernel does have a compile option for WARNing on sleeps
in sleepless contexts

> 
> > +    let flags = unsafe { bindings::local_irq_save() };
> > +
> > +    let ret = cb(IrqDisabled(PhantomData));
> > +
> > +    // SAFETY: `flags` comes from our previous call to local_irq_save
> > +    unsafe { bindings::local_irq_restore(flags) };
> 
> Just to make sure, this function only enables interrupts, if they were
> enabled before the call to `local_irq_save` above, right?

Correct - `local_irq_save()` only saves the CPU's current IRQ flags. So if
interrupts were already disabled in the context we call `local_irq_save()`, we
end up restoring the same IRQ-disabled flags on the processor when we get to
`local_irq_restore()`. This is also why the closure interface for this is
necessary - to ensure that nested interrupt flag saves are always undone in
the reverse order to ensure this assumption always holds.

> 
> ---
> Cheers,
> Benno
> 
> > +
> > +    ret
> > +}
> > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> > index e6b7d3a80bbce..37835ccd51087 100644
> > --- a/rust/kernel/lib.rs
> > +++ b/rust/kernel/lib.rs
> > @@ -36,6 +36,7 @@
> >  pub mod firmware;
> >  pub mod init;
> >  pub mod ioctl;
> > +pub mod irq;
> >  #[cfg(CONFIG_KUNIT)]
> >  pub mod kunit;
> >  #[cfg(CONFIG_NET)]
> > --
> > 2.45.2
> > 
> 

-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat

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