Date: Fri, 15 Mar 2013 14:46:32 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Ming Lei <tom.leiming@gmail.com>
Cc: Shaohua Li <shli@kernel.org>, Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: +
	atomic-improve-atomic_inc_unless_negative-atomic_dec_unless_positive
	.patch added to -mm tree
Message-ID: <20130315134632.GA18335@redhat.com>
References: <20130314162413.GA21344@redhat.com> <CACVXFVP+fGeWS3xwBfaQDFaH7=e_4CbgCru9DJczB2vyWuUL1g@mail.gmail.com>
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On 03/15, Ming Lei wrote:
>
> On Fri, Mar 15, 2013 at 12:24 AM, Oleg Nesterov <oleg@redhat.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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