Re: [PATCH net-next v3 7/8] rust: Add read_poll_timeout functions

[Alice Ryhl <aliceryhl@google.com>]

On Wed, Oct 16, 2024 at 5:54 AM FUJITA Tomonori
<fujita.tomonori@gmail.com> wrote:
>
> Add read_poll_timeout functions which poll periodically until a
> condition is met or a timeout is reached.
>
> C's read_poll_timeout (include/linux/iopoll.h) is a complicated macro
> and a simple wrapper for Rust doesn't work. So this implements the
> same functionality in Rust.
>
> The C version uses usleep_range() while the Rust version uses
> fsleep(), which uses the best sleep method so it works with spans that
> usleep_range() doesn't work nicely with.
>
> Unlike the C version, __might_sleep() is used instead of might_sleep()
> to show proper debug info; the file name and line
> number. might_resched() could be added to match what the C version
> does but this function works without it.
>
> For the proper debug info, readx_poll_timeout() is implemented as a
> macro.
>
> readx_poll_timeout() 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.
>
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
> Link: https://rust-for-linux.com/klint [1]
>
> Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>

Duplicated SOB? This should just be:

Link: https://rust-for-linux.com/klint [1]
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>

> +/// Polls periodically until a condition is met or a timeout is reached.
> +///
> +/// Public but hidden since it should only be used from public macros.
> +#[doc(hidden)]
> +pub fn read_poll_timeout<Op, Cond, T: Copy>(
> +    mut op: Op,
> +    cond: Cond,
> +    sleep_delta: Delta,
> +    timeout_delta: Delta,
> +    sleep_before_read: bool,
> +) -> Result<T>
> +where
> +    Op: FnMut() -> Result<T>,
> +    Cond: Fn(T) -> bool,
> +{
> +    let timeout = Ktime::ktime_get() + timeout_delta;
> +    let sleep = !sleep_delta.is_zero();
> +
> +    if sleep_before_read && sleep {
> +        fsleep(sleep_delta);
> +    }

You always pass `false` for `sleep_before_read` so perhaps just remove
this and the argument entirely?

> +        if cond(val) {
> +            break val;
> +        }

This breaks out to another cond(val) check below. Perhaps just `return
Ok(val)` here to avoid the double condition check?

> +        if !timeout_delta.is_zero() && Ktime::ktime_get() > timeout {
> +            break op()?;
> +        }

Shouldn't you just return `Err(ETIMEDOUT)` here? I don't think you're
supposed to call `op` twice without a sleep in between.

> +        if sleep {
> +            fsleep(sleep_delta);
> +        }
> +        // SAFETY: FFI call.
> +        unsafe { bindings::cpu_relax() }

Should cpu_relax() be in an else branch? Also, please add a safe
wrapper function around cpu_relax.

> +macro_rules! readx_poll_timeout {
> +    ($op:expr, $cond:expr, $sleep_delta:expr, $timeout_delta:expr) => {{
> +        #[cfg(CONFIG_DEBUG_ATOMIC_SLEEP)]
> +        if !$sleep_delta.is_zero() {
> +            // SAFETY: FFI call.
> +            unsafe {
> +                $crate::bindings::__might_sleep(
> +                    ::core::file!().as_ptr() as *const i8,
> +                    ::core::line!() as i32,
> +                )
> +            }
> +        }

It could be nice to introduce a might_sleep macro that does this
internally? Then we can reuse this logic in other places.

Alice