linux-kernel - Re: [PATCH v9 7/8] rust: Add read_poll

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Message-Id: <20250129.153120.170416639063888853.fujita.tomonori@gmail.com>
Date: Wed, 29 Jan 2025 15:31:20 +0900 (JST)
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: fujita.tomonori@...il.com
Cc: gary@...yguo.net, linux-kernel@...r.kernel.org,
 rust-for-linux@...r.kernel.org, netdev@...r.kernel.org, andrew@...n.ch,
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Subject: Re: [PATCH v9 7/8] rust: Add read_poll_timeout functions

On Tue, 28 Jan 2025 15:29:57 +0900 (JST)
FUJITA Tomonori <fujita.tomonori@...il.com> wrote:

> On Tue, 28 Jan 2025 08:49:37 +0800
> Gary Guo <gary@...yguo.net> wrote:
> 
>> On Mon, 27 Jan 2025 15:31:47 +0900 (JST)
>> FUJITA Tomonori <fujita.tomonori@...il.com> wrote:
>> 
>>> On Mon, 27 Jan 2025 11:46:46 +0800
>>> Gary Guo <gary@...yguo.net> wrote:
>>> 
>>> >> +#[track_caller]
>>> >> +pub fn read_poll_timeout<Op, Cond, T: Copy>(
>>> >> +    mut op: Op,
>>> >> +    mut cond: Cond,
>>> >> +    sleep_delta: Delta,
>>> >> +    timeout_delta: Delta,
>>> >> +) -> Result<T>
>>> >> +where
>>> >> +    Op: FnMut() -> Result<T>,
>>> >> +    Cond: FnMut(&T) -> bool,
>>> >> +{
>>> >> +    let start = Instant::now();
>>> >> +    let sleep = !sleep_delta.is_zero();
>>> >> +    let timeout = !timeout_delta.is_zero();
>>> >> +
>>> >> +    if sleep {
>>> >> +        might_sleep(Location::caller());
>>> >> +    }
>>> >> +
>>> >> +    loop {
>>> >> +        let val = op()?;
>>> >> +        if cond(&val) {
>>> >> +            // Unlike the C version, we immediately return.
>>> >> +            // We know the condition is met so we don't need to check again.
>>> >> +            return Ok(val);
>>> >> +        }
>>> >> +        if timeout && start.elapsed() > timeout_delta {  
>>> > 
>>> > Re-reading this again I wonder if this is the desired behaviour? Maybe
>>> > a timeout of 0 should mean check-once instead of no timeout. The
>>> > special-casing of 0 makes sense in C but in Rust we should use `None`
>>> > to mean it instead?  
>>> 
>>> It's the behavior of the C version; the comment of this function says:
>>> 
>>> * @timeout_us: Timeout in us, 0 means never timeout
>>> 
>>> You meant that waiting for a condition without a timeout is generally
>>> a bad idea? If so, can we simply return EINVAL for zero Delta?
>>> 
>> 
>> No, I think we should still keep the ability to represent indefinite
>> wait (no timeout) but we should use `None` to represent this rather
>> than `Delta::ZERO`.
>> 
>> I know that we use 0 to mean indefinite wait in C, I am saying that
>> it's not the most intuitive way to represent in Rust.
>> 
>> Intuitively, a timeout of 0 should be closer to a timeout of 1 and thus
>> should mean "return with ETIMEDOUT immedidately" rather than "wait
>> forever".
>> 
>> In C since we don't have a very good sum type support, so we
>> special case 0 to be the special value to represent indefinite wait,
>> but I don't think we need to repeat this in Rust.
> 
> Understood, thanks. How about the following code?
> 
> +#[track_caller]
> +pub fn read_poll_timeout<Op, Cond, T: Copy>(

Oops, `Copy` should be dropped:

+pub fn read_poll_timeout<Op, Cond, T>(


> +    mut op: Op,
> +    mut cond: Cond,
> +    sleep_delta: Delta,
> +    timeout_delta: Option<Delta>,
> +) -> Result<T>
> +where
> +    Op: FnMut() -> Result<T>,
> +    Cond: FnMut(&T) -> bool,
> +{
> +    let start = Instant::now();
> +    let sleep = !sleep_delta.is_zero();
> +
> +    if sleep {
> +        might_sleep(Location::caller());
> +    }
> +
> +    loop {
> +        let val = op()?;
> +        if cond(&val) {
> +            // Unlike the C version, we immediately return.
> +            // We know the condition is met so we don't need to check again.
> +            return Ok(val);
> +        }
> +        if let Some(timeout_delta) = timeout_delta {
> +            if start.elapsed() > timeout_delta {
> +                // Unlike the C version, we immediately return.
> +                // We have just called `op()` so we don't need to call it again.
> +                return Err(ETIMEDOUT);
> +            }
> +        }
> +        if sleep {
> +            fsleep(sleep_delta);
> +        }
> +        // fsleep() could be busy-wait loop so we always call cpu_relax().
> +        cpu_relax();
> +    }
> +}
>