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

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Message-ID: <875xqaw92u.ffs@tglx>
Date: Wed, 02 Oct 2024 22:20:25 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Lyude Paul <lyude@...hat.com>, rust-for-linux@...r.kernel.org
Cc: Danilo Krummrich <dakr@...hat.com>, airlied@...hat.com, Ingo Molnar
 <mingo@...hat.com>, Will Deacon <will@...nel.org>, Waiman Long
 <longman@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 linux-kernel@...r.kernel.org, Benno Lossin <benno.lossin@...ton.me>,
 Daniel Almeida <daniel.almeida@...labora.com>, Gary Guo
 <gary@...yguo.net>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor
 <alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, Boqun
 Feng <boqun.feng@...il.com>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>,
 Andreas Hindborg <a.hindborg@...sung.com>, Alice Ryhl
 <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, FUJITA Tomonori
 <fujita.tomonori@...il.com>, Valentin Obst <kernel@...entinobst.de>
Subject: Re: [PATCH v6 1/3] rust: Introduce irq module

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?

> +/// 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?

> +    // 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