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

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Message-ID: <3535d832-7980-4c95-a2f7-b6d90b172173@de.bosch.com>
Date: Thu, 12 Sep 2024 08:06:24 +0200
From: Dirk Behme <dirk.behme@...bosch.com>
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>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor
	<alex.gaynor@...il.com>, Wedson Almeida Filho <wedsonaf@...il.com>, "Boqun
 Feng" <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, Benno Lossin
	<benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...sung.com>, "Alice
 Ryhl" <aliceryhl@...gle.com>, Asahi Lina <lina@...hilina.net>, Valentin Obst
	<kernel@...entinobst.de>
Subject: Re: [PATCH v4 1/3] rust: Introduce irq module

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@...hat.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@...hat.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 ...".

Dirk