Re: [PATCH v3 05/10] lib/refcount: Improve performance of generic REFCOUNT_FULL code

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 09, 2019 at 05:44:33PM +0100, Will Deacon wrote:

> > > @@ -224,26 +208,19 @@ static inline void refcount_inc(refcount_t *r)
> > >   */
> > >  static inline __must_check bool refcount_sub_and_test(int i, refcount_t *r)
> > >  {
> > > +	int old = atomic_fetch_sub_release(i, &r->refs);
> > >  
> > > +	if (old == i) {
> > >  		smp_acquire__after_ctrl_dep();
> > >  		return true;
> > >  	}
> > >  
> > > +	if (unlikely(old - i < 0)) {
> > > +		refcount_set(r, REFCOUNT_SATURATED);
> > > +		WARN_ONCE(1, "refcount_t: underflow; use-after-free.\n");
> > > +	}
> > 
> > I'm failing to see how this preserves REFCOUNT_SATURATED for
> > non-underflow. AFAICT this should have:
> > 
> > 	if (unlikely(old == REFCOUNT_SATURATED || old - i < 0))
> 
> Well spotted! I think we just want:
> 
> 	if (unlikely(old < 0 || old - i < 0))
> 
> here, which is reassuringly similar to the logic in refcount_add() and
> refcount_add_not_zero().

Oh indeed, I missed that saturated was negative. That should work.

> > > +	return false;
> > >  }
> > >  
> > >  /**
> > > @@ -276,9 +253,13 @@ static inline __must_check bool refcount_dec_and_test(refcount_t *r)
> > >   */
> > >  static inline void refcount_dec(refcount_t *r)
> > >  {
> > > +	int old = atomic_fetch_sub_release(1, &r->refs);
> > >  
> > > +	if (unlikely(old <= 1)) {
> > 
> > Idem.
> 
> Hmm, I don't get what you mean with the one, since we're looking at the
> old value. REFCOUNT_SATURATED is negative, so it will do the right thing.

Yep, missed that.

> > > +		refcount_set(r, REFCOUNT_SATURATED);
> > > +		WARN_ONCE(1, "refcount_t: decrement hit 0; leaking memory.\n");
> > > +	}
> > > +}
> > 
> > Also, things like refcount_dec_not_one() might need fixing to preserve
> > REFCOUNT_SATURATED, because they're not expecting that value to actually
> > change, but you do!
> 
> refcount_dec_not_one() already checks for REFCOUNT_SATURATED and, in the
> case of a racing thread setting the saturated value, the cmpxchg() will
> fail if the saturated value is written after the check or the saturated
> value will overwrite the value written by the cmpxchg(). Is there another
> race that you're thinking of?

Hmm, yes. I was afraid that by not recognising SATURATED it'd go wrong,
but now that I try I can't make it go wrong.