linux-kernel - Re: [PATCH v7 11/17] refcount: introduce __refcount_{add|inc}_not_zero

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Message-ID: <CAJuCfpG-+eNvFaqpAPnk75esx88MnPbiX6wzFYbJ-ois2mkCJg@mail.gmail.com>
Date: Wed, 8 Jan 2025 07:45:02 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Vlastimil Babka <vbabka@...e.cz>, akpm@...ux-foundation.org, peterz@...radead.org, 
	liam.howlett@...cle.com, lorenzo.stoakes@...cle.com, mhocko@...e.com, 
	hannes@...xchg.org, mjguzik@...il.com, oliver.sang@...el.com, 
	mgorman@...hsingularity.net, david@...hat.com, peterx@...hat.com, 
	oleg@...hat.com, dave@...olabs.net, paulmck@...nel.org, brauner@...nel.org, 
	dhowells@...hat.com, hdanton@...a.com, hughd@...gle.com, 
	lokeshgidra@...gle.com, minchan@...gle.com, jannh@...gle.com, 
	shakeel.butt@...ux.dev, souravpanda@...gle.com, pasha.tatashin@...een.com, 
	klarasmodin@...il.com, corbet@....net, linux-doc@...r.kernel.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v7 11/17] refcount: introduce __refcount_{add|inc}_not_zero_limited

On Wed, Jan 8, 2025 at 7:06 AM Matthew Wilcox <willy@...radead.org> wrote:
>
> 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)

Thanks for the suggestions, Vlastimil and Matthew! Yes, this looks
much better. Will use it in the next version.

>
> > > +                   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);

Ack.

>