netdev - Re: [PATCH v8 4/7] rust: time: Add wrapper for fsleep function

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Message-Id: <20250117.232015.1047782190952648538.fujita.tomonori@gmail.com>
Date: Fri, 17 Jan 2025 23:20:15 +0900 (JST)
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: aliceryhl@...gle.com
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Subject: Re: [PATCH v8 4/7] rust: time: Add wrapper for fsleep function

On Fri, 17 Jan 2025 14:05:52 +0100
Alice Ryhl <aliceryhl@...gle.com> wrote:

> On Fri, Jan 17, 2025 at 10:55 AM FUJITA Tomonori
> <fujita.tomonori@...il.com> wrote:
>>
>> On Fri, 17 Jan 2025 10:13:08 +0100
>> Alice Ryhl <aliceryhl@...gle.com> wrote:
>>
>> > On Fri, Jan 17, 2025 at 10:01 AM FUJITA Tomonori
>> > <fujita.tomonori@...il.com> wrote:
>> >>
>> >> On Fri, 17 Jan 2025 16:53:26 +0900 (JST)
>> >> FUJITA Tomonori <fujita.tomonori@...il.com> wrote:
>> >>
>> >> > On Thu, 16 Jan 2025 10:27:02 +0100
>> >> > Alice Ryhl <aliceryhl@...gle.com> wrote:
>> >> >
>> >> >>> +/// This function can only be used in a nonatomic context.
>> >> >>> +pub fn fsleep(delta: Delta) {
>> >> >>> +    // The argument of fsleep is an unsigned long, 32-bit on 32-bit architectures.
>> >> >>> +    // Considering that fsleep rounds up the duration to the nearest millisecond,
>> >> >>> +    // set the maximum value to u32::MAX / 2 microseconds.
>> >> >>> +    const MAX_DURATION: Delta = Delta::from_micros(u32::MAX as i64 >> 1);
>> >> >>
>> >> >> Hmm, is this value correct on 64-bit platforms?
>> >> >
>> >> > You meant that the maximum can be longer on 64-bit platforms? 2147484
>> >> > milliseconds is long enough for fsleep's duration?
>> >> >
>> >> > If you prefer, I use different maximum durations for 64-bit and 32-bit
>> >> > platforms, respectively.
>> >>
>> >> How about the following?
>> >>
>> >> const MAX_DURATION: Delta = Delta::from_micros(usize::MAX as i64 >> 1);
>> >
>> > Why is there a maximum in the first place? Are you worried about
>> > overflow on the C side?
>>
>> Yeah, Boqun is concerned that an incorrect input (a negative value or
>> an overflow on the C side) leads to unintentional infinite sleep:
>>
>> https://lore.kernel.org/lkml/ZxwVuceNORRAI7FV@Boquns-Mac-mini.local/
> 
> Okay, can you explain in the comment that this maximum value prevents
> integer overflow inside fsleep?

Surely, how about the following?

pub fn fsleep(delta: Delta) {
    // The argument of fsleep is an unsigned long, 32-bit on 32-bit architectures.
    // Considering that fsleep rounds up the duration to the nearest millisecond,
    // set the maximum value to u32::MAX / 2 microseconds to prevent integer
    // overflow inside fsleep, which could lead to unintentional infinite sleep.
    const MAX_DURATION: Delta = Delta::from_micros(u32::MAX as i64 >> 1);