Messages in this thread | | | Date | Tue, 23 Apr 2024 14:11:22 -0700 | From | Boqun Feng <> | Subject | Re: [PATCH 2/2] rust: time: Use wrapping_sub() for Ktime::sub() |
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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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