Messages in this thread | | | Date | Mon, 11 Feb 2019 17:29:07 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] Documentation/atomic_t: Clarify signed vs unsigned |
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On Tue, Feb 12, 2019 at 09:03:55AM +0800, Boqun Feng wrote: > On Mon, Feb 11, 2019 at 06:09:43PM +0100, Peter Zijlstra wrote: > > > > Clarify the whole signed vs unsigned issue for atomic_t. > > > > There has been enough confusion on this topic to warrant a few explicit > > words I feel. > > > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> > > FWIW > > Acked-by: Boqun Feng <boqun.feng@gmail.com>
Applied, thank you!
Thanx, Paul
> Regards, > Boqun > > > --- > > Documentation/atomic_t.txt | 17 +++++++++++++++++ > > 1 file changed, 17 insertions(+) > > > > diff --git a/Documentation/atomic_t.txt b/Documentation/atomic_t.txt > > index 913396ac5824..dca3fb0554db 100644 > > --- a/Documentation/atomic_t.txt > > +++ b/Documentation/atomic_t.txt > > @@ -56,6 +56,23 @@ The 'full' API consists of (atomic64_ and atomic_long_ prefixes omitted for > > smp_mb__{before,after}_atomic() > > > > > > +TYPES (signed vs unsigned) > > +----- > > + > > +While atomic_t, atomic_long_t and atomic64_t use int, long and s64 > > +respectively (for hysterical raisins), the kernel uses -fno-strict-overflow > > +(which implies -fwrapv) and defines signed overflow to behave like > > +2s-complement. > > + > > +Therefore, an explicitly unsigned variant of the atomic ops is strictly > > +unnecessary and we can simply cast, there is no UB. > > + > > +There was a bug in UBSAN prior to GCC-8 that would generate UB warnings for > > +signed types. > > + > > +With this we also conform to the C/C++ _Atomic behaviour and things like > > +P1236R1. > > + > > > > SEMANTICS > > ---------
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