[RFC PATCH] lib: Introduce generic __cmpxchg_u64() and use it where needed

[Peter Zijlstra]

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).