linux-kernel - Re: [PATCH v1 2/2] rust: Add read_poll_timeout

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Message-Id: <33725E09-3994-4306-AD4F-4A0253261DE1@collabora.com>
Date: Tue, 26 Aug 2025 13:59:46 -0300
From: Daniel Almeida <daniel.almeida@...labora.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: FUJITA Tomonori <fujita.tomonori@...il.com>,
 a.hindborg@...nel.org,
 alex.gaynor@...il.com,
 ojeda@...nel.org,
 aliceryhl@...gle.com,
 anna-maria@...utronix.de,
 bjorn3_gh@...tonmail.com,
 boqun.feng@...il.com,
 frederic@...nel.org,
 gary@...yguo.net,
 jstultz@...gle.com,
 linux-kernel@...r.kernel.org,
 lossin@...nel.org,
 lyude@...hat.com,
 rust-for-linux@...r.kernel.org,
 sboyd@...nel.org,
 tglx@...utronix.de,
 tmgross@...ch.edu,
 acourbot@...dia.com
Subject: Re: [PATCH v1 2/2] rust: Add read_poll_timeout_atomic function



> On 26 Aug 2025, at 11:12, Danilo Krummrich <dakr@...nel.org> wrote:
> 
> On Thu Aug 21, 2025 at 5:57 AM CEST, FUJITA Tomonori wrote:
>> +pub fn read_poll_timeout_atomic<Op, Cond, T>(
>> +    mut op: Op,
>> +    mut cond: Cond,
>> +    delay_delta: Delta,
>> +    timeout_delta: Delta,
>> +) -> Result<T>
>> +where
>> +    Op: FnMut() -> Result<T>,
>> +    Cond: FnMut(&T) -> bool,
>> +{
>> +    let mut left_ns = timeout_delta.as_nanos();
>> +    let delay_ns = delay_delta.as_nanos();
>> +
>> +    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 left_ns < 0 {
>> +            // 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 !delay_delta.is_zero() {
>> +            udelay(delay_delta);
>> +            left_ns -= delay_ns;
>> +        }
>> +
>> +        cpu_relax();
>> +        left_ns -= 1;
> 
> How do we know that each iteration costs 1ns? To make it even more obvious, we
> don't control the implementation of cond(). Shouldn't we use ktime for this?

I don’t think ktime can be used from an atomic context?

— Daniel