Re: [PATCH v6 1/3] rust: Introduce irq module

[Lyude Paul <lyude@redhat.com>]

On Wed, 2024-10-02 at 22:20 +0200, Thomas Gleixner wrote:
> On Mon, Sep 16 2024 at 17:28, Lyude Paul wrote:
> >  rust/helpers/helpers.c |  1 +
> >  rust/helpers/irq.c     | 22 ++++++++++
> >  rust/kernel/irq.rs     | 96 ++++++++++++++++++++++++++++++++++++++++++
> 
> irq is a patently bad name for this as it might get confused or conflict
> with actual interrupt related functions irq_.....
> 
> The C naming is not ideal either but it's all about the CPU local
> interrupt enable/disable, while irq_*() is related to actual interrupt
> handling and chips.
> 
> So can we please have some halfways sensible mapping to the C namings?

I'm fine with renaming this, looking at the naming of the C functions perhaps
this would be preferrable?

with_local_irqs_disabled
LocalIrqsDisabled

> 
> > +/// Run the closure `cb` with interrupts disabled on the local CPU.
> > +///
> > +/// This disables interrupts, creates an [`IrqDisabled`] token and passes it to `cb`. The previous
> > +/// interrupt state will be restored once the closure completes. 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
> > +/// }
> > +///
> > +/// // Disables interrupts, their previous state will be restored once the 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() });
> 
> What's the point of the IrqDisabled::new() here? The above just disabled
> them, no?

TBH I kind of agree, the original version of this patch series didn't actually
call the constructor here and just created the token directly - but IMHO I'm
not sure how necessary it is when we can see the call for disabling right
above.

> 
> > +    // Confirm that IRQs are still disabled now that the callback has finished
> > +    // SAFETY: FFI call with no special requirements
> > +    debug_assert!(unsafe { bindings::irqs_disabled() });
> 
> And here you open code the check which is in IrqDisabled::new()
> 
> So I'd rather see this as:
> 
>    token = unsafe { IrqDisabled::new() };
>    let ret = cb(token);
>    assert_valid(token);
> 
> I might misunderstand rust here, but the provided code does not make
> sense to me.
> 
> Thanks,
> 
>         tglx
> 

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

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