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

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Message-Id: <20250127.153147.1789884009486719687.fujita.tomonori@gmail.com>
Date: Mon, 27 Jan 2025 15:31:47 +0900 (JST)
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: gary@...yguo.net
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Subject: Re: [PATCH v9 7/8] rust: Add read_poll_timeout functions

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?