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

[David Laight <david.laight.linux@gmail.com>]

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.

	David