Re: [PATCH v1 2/2] rust: task: add Rust version of might_sleep()

[Alice Ryhl <aliceryhl@google.com>]

On Sun, Apr 06, 2025 at 08:07:18PM +0900, FUJITA Tomonori wrote:
> Adds a helper function equivalent to the C's might_sleep(), which
> serves as a debugging aid and a potential scheduling point.
> 
> Note that this function can only be used in a nonatomic context.
> 
> This will be used by Rust version of read_poll_timeout().
> 
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> ---
>  rust/helpers/task.c |  6 ++++++
>  rust/kernel/task.rs | 26 ++++++++++++++++++++++++++
>  2 files changed, 32 insertions(+)
> 
> diff --git a/rust/helpers/task.c b/rust/helpers/task.c
> index 31c33ea2dce6..2c85bbc2727e 100644
> --- a/rust/helpers/task.c
> +++ b/rust/helpers/task.c
> @@ -1,7 +1,13 @@
>  // SPDX-License-Identifier: GPL-2.0
>  
> +#include <linux/kernel.h>
>  #include <linux/sched/task.h>
>  
> +void rust_helper_might_resched(void)
> +{
> +	might_resched();
> +}
> +
>  struct task_struct *rust_helper_get_current(void)
>  {
>  	return current;
> diff --git a/rust/kernel/task.rs b/rust/kernel/task.rs
> index 9e6f6854948d..1f0156b38ab5 100644
> --- a/rust/kernel/task.rs
> +++ b/rust/kernel/task.rs
> @@ -380,3 +380,29 @@ fn eq(&self, other: &Kuid) -> bool {
>  }
>  
>  impl Eq for Kuid {}
> +
> +/// Annotation for functions that can sleep.
> +///
> +/// Equivalent to the C side [`might_sleep()`], this function serves as
> +/// a debugging aid and a potential scheduling point.
> +///
> +/// This function can only be used in a nonatomic context.
> +#[track_caller]
> +#[inline]
> +pub fn might_sleep() {
> +    #[cfg(CONFIG_DEBUG_ATOMIC_SLEEP)]
> +    {
> +        let loc = core::panic::Location::caller();
> +        // SAFETY: FFI call.

Overall this looks okay to me, but this safety comment could be
improved. This being an FFI call is not the reason *why* it is safe to
make this call.

// SAFETY: `file.as_ptr()` is valid for reading for `file.len()` bytes.

And I might separate the file into a separate variable for clarity:
let loc = core::panic::Location::caller();
let file = loc.file();

> +        unsafe {
> +            crate::bindings::__might_sleep_precision(
> +                loc.file().as_ptr().cast(),
> +                loc.file().len() as i32,
> +                loc.line() as i32,
> +            )
> +        }
> +    }
> +
> +    // SAFETY: FFI call.
> +    unsafe { crate::bindings::might_resched() }

And here you can say
// SAFETY: Always safe to call.

Alice