Re: [PATCH v7 11/17] refcount: introduce __refcount_{add|inc}_not_zero_limited

[Matthew Wilcox]

On Wed, Jan 08, 2025 at 10:16:04AM +0100, Vlastimil Babka wrote:
> >  static inline __must_check __signed_wrap
> > -bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
> > +bool __refcount_add_not_zero_limited(int i, refcount_t *r, int *oldp,
> > +				     int limit)
> >  {
> >  	int old = refcount_read(r);
> >  
> >  	do {
> >  		if (!old)
> >  			break;
> > +		if (limit && old + i > limit) {
> 
> Should this be e.g. "old > limit - i" to avoid overflow and false negative
> if someone sets limit close to INT_MAX?

Although 'i' might also be INT_MAX, whereas we know that old < limit.
So "i > limit - old" is the correct condition to check, IMO.

I'd further suggest that using a limit of 0 to mean "unlimited" introduces
an unnecessary arithmetic operation.  Make 'limit' inclusive instead
of exclusive, pass INT_MAX instead of 0, and Vlastimil's suggestion,
and this becomes:

		if (i > limit - old)

> > +			if (oldp)
> > +				*oldp = old;
> > +			return false;
> > +		}
> >  	} while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));

...

> > +static inline __must_check __signed_wrap
> > +bool __refcount_add_not_zero(int i, refcount_t *r, int *oldp)
> > +{
> > +	return __refcount_add_not_zero_limited(i, r, oldp, 0);

Just to be clear, this becomes:

	return __refcount_add_not_zero_limited(i, r, oldp, INT_MAX);