Messages in this thread | | | Date | Mon, 11 Feb 2019 17:28:00 +0000 | From | Will Deacon <> | Subject | Re: [PATCH] Documentation/atomic_t: Clarify signed vs unsigned |
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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> > --- > 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. > +
Acked-by: Will Deacon <will.deacon@arm.com>
Will
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