linux-kernel - Re: [PATCH v9 5/8] rust: time: Add wrapper for fsleep() function

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Message-ID: <EED019B1-8DEB-43BF-8F59-1A71520F5ABB@kloenk.dev>
Date: Tue, 28 Jan 2025 11:37:41 +0100
From: Fiona Behrens <me@...enk.dev>
To: FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
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Subject: Re: [PATCH v9 5/8] rust: time: Add wrapper for fsleep() function



On 25 Jan 2025, at 11:18, FUJITA Tomonori wrote:

> Add a wrapper for fsleep(), flexible sleep functions in
> include/linux/delay.h which typically deals with hardware delays.
>
> The kernel supports several sleep functions to handle various lengths
> of delay. This adds fsleep(), automatically chooses the best sleep
> method based on a duration.
>
> sleep functions including fsleep() belongs to TIMERS, not
> TIMEKEEPING. They are maintained separately. rust/kernel/time.rs is an
> abstraction for TIMEKEEPING. To make Rust abstractions match the C
> side, add rust/kernel/time/delay.rs for this wrapper.
>
> fsleep() can only be used in a nonatomic context. This requirement is
> not checked by these abstractions, but it is intended that klint [1]
> or a similar tool will be used to check it in the future.
>
> Link: https://rust-for-linux.com/klint [1]
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@...il.com>

One question below, but fine with this as well

Reviewed-by: Fiona Behrens <me@...enk.dev>

> ---
>  rust/helpers/helpers.c    |  1 +
>  rust/helpers/time.c       |  8 +++++++
>  rust/kernel/time.rs       |  2 ++
>  rust/kernel/time/delay.rs | 49 +++++++++++++++++++++++++++++++++++++++
>  4 files changed, 60 insertions(+)
>  create mode 100644 rust/helpers/time.c
>  create mode 100644 rust/kernel/time/delay.rs
>
(..)
> diff --git a/rust/kernel/time/delay.rs b/rust/kernel/time/delay.rs
> new file mode 100644
> index 000000000000..02b8731433c7
> --- /dev/null
> +++ b/rust/kernel/time/delay.rs
> @@ -0,0 +1,49 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Delay and sleep primitives.
> +//!
> +//! This module contains the kernel APIs related to delay and sleep that
> +//! have been ported or wrapped for usage by Rust code in the kernel.
> +//!
> +//! C header: [`include/linux/delay.h`](srctree/include/linux/delay.h).
> +
> +use super::Delta;
> +use crate::ffi::c_ulong;
> +
> +/// Sleeps for a given duration at least.
> +///
> +/// Equivalent to the C side [`fsleep()`], flexible sleep function,
> +/// which automatically chooses the best sleep method based on a duration.
> +///
> +/// `delta` must be within `[0, i32::MAX]` microseconds;
> +/// otherwise, it is erroneous behavior. That is, it is considered a bug
> +/// to call this function with an out-of-range value, in which case the function
> +/// will sleep for at least the maximum value in the range and may warn
> +/// in the future.
> +///
> +/// The behavior above differs from the C side [`fsleep()`] for which out-of-range
> +/// values mean "infinite timeout" instead.
> +///
> +/// This function can only be used in a nonatomic context.
> +///
> +/// [`fsleep`]: https://docs.kernel.org/timers/delay_sleep_functions.html#c.fsleep
> +pub fn fsleep(delta: Delta) {
> +    // The maximum value is set to `i32::MAX` microseconds to prevent integer
> +    // overflow inside fsleep, which could lead to unintentional infinite sleep.
> +    const MAX_DELTA: Delta = Delta::from_micros(i32::MAX as i64);
> +
> +    let delta = if (Delta::ZERO..=MAX_DELTA).contains(&delta) {
> +        delta
> +    } else {
> +        // TODO: Add WARN_ONCE() when it's supported.
> +        MAX_DELTA
> +    };

Did you try that with std::cmp::Ord you derived on Delta? This `.contains` looks a bit weird, maybe it also works with `delta <= MAX_DELTA`?

> +
> +    // SAFETY: It is always safe to call `fsleep()` with any duration.
> +    unsafe {
> +        // Convert the duration to microseconds and round up to preserve
> +        // the guarantee; `fsleep()` sleeps for at least the provided duration,
> +        // but that it may sleep for longer under some circumstances.
> +        bindings::fsleep(delta.as_micros_ceil() as c_ulong)
> +    }
> +}
> -- 
> 2.43.0