On Thu, Nov 01, 2018 at 06:18:46PM +0100, Peter Zijlstra wrote: > On Thu, Nov 01, 2018 at 10:01:46AM -0700, Paul E. McKenney wrote: > > On Thu, Nov 01, 2018 at 05:32:12PM +0100, Peter Zijlstra wrote: > > > 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 :-) > > > > Even the C standard defines 2s complement for atomics. > > Ooh good to know. Must be some mistake, right? ;-) > > Just not for > > normal arithmetic, where yes, signed overflow is UB. And yes, I do > > know about -fwrapv, but I would like to avoid at least some copy-pasta > > UB from my kernel code to who knows what user-mode environment. :-/ > > > > At least where it is reasonably easy to do so. > > Fair enough I suppose; I just always make sure to include the same > -fknobs for the userspace thing when I lift code. Agreed! But when it is other people lifting the code... > > And there is a push to define C++ signed arithmetic as 2s complement, > > but there are still 1s complement systems with C compilers. Just not > > C++ compilers. Legacy... > > *groan*; how about those ancient hardwares keep using ancient compilers > and we all move on to the 70s :-) Hey!!! Some of that 70s (and 60s!) 1s-complement hardware helped pay my way through university the first time around!!! ;-) Though where it once filled a room it is now on a single small chip. Go figure... > > > 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. > > > > UBSAN gave me a signed-overflow warning a few days ago. Which I have > > fixed, even though 2s complement did the right thing. I am also taking > > advantage of the change to use better naming. > > Oh too many *SANs I suppose; and yes, if you can make the code better, > why not. Yeah, when INT_MIN was confined to a single function, no problem. But thanks to the RCU flavor consolidation, it has to be spread out a bit more... Plus there is now INT_MAX, INT_MAX/2, ... > > > > 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. > > > > Color me confused. My 5a581b367b5d is from 2013. Or is "Paul" instead > > intended to mean Paul Mackerras, who happens to be on CC? > > Paul Burton I think, on a part of the thread before we joined :-) Couldn't be bothered to look up the earlier part of the thread. Getting lazy in my old age. ;-) Thanx, Paul
