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

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Message-Id: <20250128.152957.202492012529466658.fujita.tomonori@gmail.com>
Date: Tue, 28 Jan 2025 15:29:57 +0900 (JST)
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: gary@...yguo.net
Cc: fujita.tomonori@...il.com, 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 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>(
+    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();
+    }
+}