On Thu, Nov 01, 2018 at 03:22:15PM +0000, Trond Myklebust wrote: > On Thu, 2018-11-01 at 15:59 +0100, Peter Zijlstra wrote: > > 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). > > Fair enough, but there have also been bugfixes to explicitly fix unsafe > C standards assumptions for signed integers. See, for instance commit > 5a581b367b5d "jiffies: Avoid undefined behavior from signed overflow" > from Paul McKenney. Yes, I feel Paul has been to too many C/C++ committee meetings and got properly paranoid. Which isn't always a bad thing :-) But for us using -fno-strict-overflow which actually defines signed overflow, I myself am really not worried. I'm also not sure if KASAN has been taught about this, or if it will still (incorrectly) warn about UB for signed types. > Anyhow, if the atomic maintainers are willing to stand up and state for > the record that the atomic counters are guaranteed to wrap modulo 2^n > just like unsigned integers, then I'm happy to take Paul's patch. I myself am certainly relying on it.
