Re: [PATCH 2/2] rust: time: Use wrapping_sub() for Ktime::sub()

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, May 09, 2024 at 02:14:16PM +0200, Thomas Gleixner wrote:
> On Tue, Apr 23 2024 at 14:11, Boqun Feng wrote:
> > On Thu, Apr 11, 2024 at 04:08:01PM -0700, Boqun Feng wrote:
> >> Currently since Rust code is compiled with "-Coverflow-checks=y", so a
> >> normal substraction may be compiled as an overflow checking and panic
> >> if overflow happens:
> >> 
> >>         subq    %rsi, %rdi
> >>         jo      .LBB0_2
> >>         movq    %rdi, %rax
> >>         retq
> >> .LBB0_2:
> >>         pushq   %rax
> >>         leaq    str.0(%rip), %rdi
> >>         leaq    .L__unnamed_1(%rip), %rdx
> >>         movl    $33, %esi
> >>         callq   *core::panicking::panic::h59297120e85ea178@GOTPCREL(%rip)
> >> 
> >> although overflow detection is nice to have, however this makes
> >> `Ktime::sub()` behave differently than `ktime_sub()`, moreover it's not
> >> clear that the overflow checking is helpful, since for example, the
> >> current binder usage[1] doesn't have the checking.
> >
> > Ping. Thomas, John and Stepthen. Could you take a look at this, and the
> > discussion between Miguel and me? The key question is the behavior when
> > ktime_sub() hits a overflow, I think. Thanks!
> 
> In principle ktime_sub() should not overflow for regular use cases.
> 
> If the binder example overflows the substraction, then something is
> seriously wrong. Though in that case as it's only for debug purposes
> panicing would be totally counter productive. A warning might be
> appropriate though.
> 

Thanks! Looks to me, a reasonable implementation would be calling
`overflowing_sub`[1] and print a warning if overflow happens:

	<in the implementation of Sub trait of Ktime>

	let (ret, overflow) = self.inner.overflowing_sub(other.inner);

	if overflow {
		pr_err!("Overflow on Ktime: {} - {}\n", self.inner, other.inner);
	}

	return ret;

I will use this as a new version for more reviews.

[1]: https://doc.rust-lang.org/std/primitive.i64.html#method.overflowing_sub

Regards,
Boqun

> Thanks,
> 
>         tglx
> 
>