linux-kernel - Re: [PATCH v9 10/17] refcount: introduce __refcount_{add|inc}_not_zero

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Message-ID: <CAJuCfpGtv4DQ-2x791e6qbXzKwRLX4uAw2Mh5++s2eOiez9DfA@mail.gmail.com>
Date: Sat, 11 Jan 2025 09:11:52 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Hillf Danton <hdanton@...a.com>
Cc: akpm@...ux-foundation.org, peterz@...radead.org, willy@...radead.org, 
	hannes@...xchg.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	kernel-team@...roid.com
Subject: Re: [PATCH v9 10/17] refcount: introduce __refcount_{add|inc}_not_zero_limited

On Sat, Jan 11, 2025 at 4:13 AM Hillf Danton <hdanton@...a.com> wrote:
>
> On Sat, 11 Jan 2025 01:59:41 -0800 Suren Baghdasaryan <surenb@...gle.com>
> > On Fri, Jan 10, 2025 at 10:32 PM Hillf Danton <hdanton@...a.com> wrote:
> > > On Fri, 10 Jan 2025 20:25:57 -0800 Suren Baghdasaryan <surenb@...gle.com>
> > > > -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 (statically_true(limit == INT_MAX))
> > > > +                     continue;
> > > > +
> > > > +             if (i > limit - old) {
> > > > +                     if (oldp)
> > > > +                             *oldp = old;
> > > > +                     return false;
> > > > +             }
> > > >       } while (!atomic_try_cmpxchg_relaxed(&r->refs, &old, old + i));
> > >
> > > The acquire version should be used, see atomic_long_try_cmpxchg_acquire()
> > > in kernel/locking/rwsem.c.
> >
> > This is how __refcount_add_not_zero() is already implemented and I'm
> > only adding support for a limit. If you think it's implemented wrong
> > then IMHO it should be fixed separately.
> >
> Two different things - refcount has nothing to do with locking at the
> first place, while what you are adding to the mm directory is something
> that replaces rwsem, so from the locking POV you have to mark the
> boundaries of the locking section.

I see your point. I think it's a strong argument to use atomic
directly instead of refcount for this locking. I'll try that and see
how it looks. Thanks for the feedback!

>
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