Re: [PATCH v9 10/17] refcount: introduce __refcount_{add|inc}_not_zero_limited - clang 17.0.1 bug

David Laight
2025-01-12T11:38:00Z

On Sat, 11 Jan 2025 22:50:16 +0000
David Laight <david.laight.linux@gmail.com> wrote:

> On Sat, 11 Jan 2025 22:19:39 +0000
> David Laight <david.laight.linux@gmail.com> wrote:
>
> > On Sat, 11 Jan 2025 10:30:40 -0800
> > "Paul E. McKenney" <paulmck@kernel.org> wrote:
> >
> > > On Sat, Jan 11, 2025 at 12:39:00PM +0000, David Laight wrote:
> > > > On Fri, 10 Jan 2025 20:25:57 -0800
> > > > Suren Baghdasaryan <surenb@google.com> wrote:
> > > >
> > > > > Introduce functions to increase refcount but with a top limit above which
> > > > > they will fail to increase (the limit is inclusive). Setting the limit to
> > > > > INT_MAX indicates no limit.
> > > >
> > > > This function has never worked as expected!
> > > > I've removed the update and added in the rest of the code.
> > > >
> > > > > diff --git a/include/linux/refcount.h b/include/linux/refcount.h
> > > > > index 35f039ecb272..5072ba99f05e 100644
> > > > > --- a/include/linux/refcount.h
> > > > > +++ b/include/linux/refcount.h
> > > > > @@ -137,13 +137,23 @@ static inline unsigned int refcount_read(const refcount_t *r)
> > > > > }
> > > > >
> > > > > static inline __must_check __signed_wrap
> > > > > -bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
> > > > > {
> > > > > int old = refcount_read(r);
> > > > >
> > > > > do {
> > > > > if (!old)
> > > > > break;
> > > > >
> > > > > } while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
> > > > >
> > > > > if (oldp)
> > > > > *oldp = old;
> > > > ?
> > > > > if (unlikely(old < 0 || old + i < 0))
> > > > > refcount_warn_saturate(r, REFCOUNT_ADD_NOT_ZERO_OVF);
> > > > >
> > > > > return old;
> > > > > }
> > > >
> > > > The saturate test just doesn't work as expected.
> > > > In C signed integer overflow is undefined (probably so that cpu that saturate/trap
> > > > signed overflow can be conformant) and gcc uses that to optimise code.
> > > >
> > > > So if you compile (https://www.godbolt.org/z/WYWo84Weq):
> > > > int inc_wraps(int i)
> > > > {
> > > > return i < 0 || i + 1 < 0;
> > > > }
> > > > the second test is optimised away.
> > > > I don't think the kernel compiles disable this optimisation.
> > >
> > > Last I checked, my kernel compiles specified -fno-strict-overflow.
> > > What happens if you try that in godbolt?
> >
> > That does make gcc generated the wanted object code.
> > I know that compilation option has come up before, but I couldn't remember the
> > name or whether it was disabled :-(
> >
> > You do get much better object code from return (i | i + 1) < 0;
> > And that is likely to be much better still if you need a conditional jump.
>
> I've just checked some more cases (see https://www.godbolt.org/z/YoM9odTbe).
> gcc 11 onwards generates the same code code for the two expressions.
>
> Rather more worryingly clang 17.0.1 is getting this one wrong:
> return i < 0 || i + 1 < 0 ? foo(i) : bar(i);
> It ignores the 'i + 1' test even with -fno-strict-overflow.
> That is more representative of the actual code.
>
> What have I missed now?

A different optimisation :-(

>
> David
>