linux-kernel - Re: + atomic-improve-atomic_inc_unless_negative-atomic_dec_unless

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Message-ID: <20130315134632.GA18335@redhat.com>
Date:	Fri, 15 Mar 2013 14:46:32 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	Shaohua Li <shli@...nel.org>, Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: +
	atomic-improve-atomic_inc_unless_negative-atomic_dec_unless_positive
	.patch added to -mm tree

On 03/15, Ming Lei wrote:
>
> On Fri, Mar 15, 2013 at 12:24 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> >  static inline int atomic_inc_unless_negative(atomic_t *p)
> >  {
> >         int v, v1;
> > -       for (v = 0; v >= 0; v = v1) {
> > +       for (v = atomic_read(p); v >= 0; v = v1) {
> >                 v1 = atomic_cmpxchg(p, v, v + 1);
>
> Unfortunately, the above will exchange the current value even though
> it is negative, so it isn't correct.

Hmm, why? We always check "v >= 0" before we try to do
atomic_cmpxchg(old => v) ?

Oleg.

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