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

    (Cc Kees as well)

    Regards,
    Boqun

    > Therefore make `Ktime::sub()` have the same semantics as `ktime_sub()`:
    > overflow behaves like 2s-complement wrapping sub.
    >
    > Link: https://lore.kernel.org/lkml/5ac8c0d09392290be789423f0dd78a520b830fab.1682333709.git.zhangchuang3@xiaomi.com/ [1]
    > Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
    > ---
    > rust/kernel/time.rs | 4 +++-
    > 1 file changed, 3 insertions(+), 1 deletion(-)
    >
    > diff --git a/rust/kernel/time.rs b/rust/kernel/time.rs
    > index e3bb5e89f88d..3cb15d3079f4 100644
    > --- a/rust/kernel/time.rs
    > +++ b/rust/kernel/time.rs
    > @@ -77,7 +77,9 @@ impl core::ops::Sub for Ktime {
    > #[inline]
    > fn sub(self, other: Ktime) -> Ktime {
    > Self {
    > - inner: self.inner - other.inner,
    > + // Mirrors `ktime_sub()`, kernel defines signed overflow to behave like 2s-complement,
    > + // hence `wrapping_sub()` is used.
    > + inner: self.inner.wrapping_sub(other.inner),
    > }
    > }
    > }
    > --
    > 2.44.0
    >

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