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

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Message-ID: <CAJuCfpF+Hr7GW6s3WHgChTp52KgWbSm_xYEANQr+54f0TiP9Aw@mail.gmail.com>
Date: Sat, 11 Jan 2025 02:00:39 -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 1:59 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> 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.
>
> >
> > Why not use the atomic_long_t without bothering to add this limited version?
>
> The check against the limit is not only for overflow protection but
> also to avoid refcount increment when the writer bit is set. It makes
> the locking code simpler if we have a function that prevents
> refcounting when the vma is detached (vm_refcnt==0) or when it's
> write-locked (vm_refcnt<VMA_REF_LIMIT).

s / vm_refcnt<VMA_REF_LIMIT / vm_refcnt>VMA_REF_LIMIT

>
> >
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> >