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

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Date:	Fri, 15 Mar 2013 23:13:43 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Oleg Nesterov <oleg@...hat.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 Fri, Mar 15, 2013 at 9:46 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> 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) ?

Sorry, yes, you are right. But then your patch is basically same with the
previous one, isn't it?  And has same problem, see below discussion:

http://marc.info/?t=136284366900001&r=1&w=2


Thanks,
-- 
Ming Lei
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