Re: [PATCH v4 1/3] rust: Introduce irq module

[Lyude Paul <lyude@redhat.com>]

On Thu, 2024-09-12 at 08:06 +0200, Dirk Behme wrote:
> On 12.09.2024 02:55, 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@redhat.com>
> > 
> > ---
> > 
> > V2:
> > * Actually make it so that we check whether or not we have interrupts
> >    disabled with debug assertions
> > * Fix issues in the documentation (added suggestions, missing periods, made
> >    sure that all rustdoc examples compile properly)
> > * Pass IrqDisabled by value, not reference
> > * Ensure that IrqDisabled is !Send and !Sync using
> >    PhantomData<(&'a (), *mut ())>
> > * Add all of the suggested derives from Benno Lossin
> > 
> > V3:
> > * Use `impl` for FnOnce bounds in with_irqs_disabled()
> > * Use higher-ranked trait bounds for the lifetime of with_irqs_disabled()
> > * Wording changes in the documentation for the module itself
> > 
> > V4:
> > * Use the actual unsafe constructor for IrqDisabled in
> >    with_irqs_disabled()
> > * Fix comment style in with_irqs_disabled example
> > * Check before calling local_irq_restore() in with_irqs_disabled that
> >    interrupts are still disabled. 
> 
> 
> This looks correct ...
> 
> 
> > It would have been nice to do this from a
> >    Drop implementation like I hoped, but I realized rust doesn't allow that
> >    for types that implement Copy.
> > * Document that interrupts can't be re-enabled within the `cb` provided to
> >    `with_irqs_disabled`, and link to the github issue I just filed about
> >    this that describes the solution for this.
> > 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> ....
> > +/// Run the closure `cb` with interrupts disabled on the local CPU.
> > +///
> > +/// This creates an [`IrqDisabled`] token, which can be passed to functions that must be run
> > +/// without interrupts. Note that interrupts must be disabled for the entire duration of `cb`, they
> > +/// cannot be re-enabled. In the future, this may be expanded on
> > +/// [as documented here](https://github.com/Rust-for-Linux/linux/issues/1115).
> > +///
> > +/// # Examples
> > +///
> > +/// Using [`with_irqs_disabled`] to call a function that can only be called with interrupts
> > +/// disabled:
> > +///
> > +/// ```
> > +/// use kernel::irq::{IrqDisabled, with_irqs_disabled};
> > +///
> > +/// // Requiring interrupts be disabled to call a function
> > +/// fn dont_interrupt_me(_irq: IrqDisabled<'_>) {
> > +///     // When this token is available, IRQs are known to be disabled. Actions that rely on this
> > +///     // can be safely performed
> > +/// }
> > +///
> > +/// // Disabling interrupts. They'll be re-enabled once this closure completes.
> > +/// with_irqs_disabled(|irq| dont_interrupt_me(irq));
> > +/// ```
> > +#[inline]
> > +pub fn with_irqs_disabled<T>(cb: impl for<'a> FnOnce(IrqDisabled<'a>) -> T) -> T {
> > +    // SAFETY: FFI call with no special requirements
> > +    let flags = unsafe { bindings::local_irq_save() };
> > +
> > +    // SAFETY: We just disabled IRQs using `local_irq_save()`
> > +    let ret = cb(unsafe { IrqDisabled::new() });
> > +
> > +    // Confirm that IRQs are still enabled now that the callback has finished
> 
> ... so here it should be 'disabled' instead of 'enabled'? "Confirm that 
> IRQs are still disabled ...".

Yes you're right :P, will fix this on the next respin

> 
> Dirk
> 

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

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