On Thu, Nov 01, 2018 at 01:18:46PM +0000, Mark Rutland wrote: > > My one question (and the reason why I went with cmpxchg() in the first > > place) would be about the overflow behaviour for atomic_fetch_inc() and > > friends. I believe those functions should be OK on x86, so that when we > > overflow the counter, it behaves like an unsigned value and wraps back > > around. Is that the case for all architectures? > > > > i.e. are atomic_t/atomic64_t always guaranteed to behave like u32/u64 > > on increment? > > > > I could not find any documentation that explicitly stated that they > > should. > > Peter, Will, I understand that the atomic_t/atomic64_t ops are required > to wrap per 2's-complement. IIUC the refcount code relies on this. > > Can you confirm? There is quite a bit of core code that hard assumes 2s-complement. Not only for atomics but for any signed integer type. Also see the kernel using -fno-strict-overflow which implies -fwrapv, which defines signed overflow to behave like 2s-complement (and rids us of that particular UB).
