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

[Peter Zijlstra <peterz@infradead.org>]

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.