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

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Date:	Fri, 15 Mar 2013 17:51:31 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Ming Lei <tom.leiming@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...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 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?

Sure, the logic is the same, just the patch (and the code) looks simpler
and more understandable.

> And has same problem, see below discussion:
>
> http://marc.info/?t=136284366900001&r=1&w=2

The lack of the barrier?

I thought about this, this should be fine? atomic_add_unless() has the same
"problem", but this is documented in atomic_ops.txt:

	atomic_add_unless requires explicit memory barriers around the operation
	unless it fails (returns 0).

I thought that atomic_add_unless_negative() should have the same
guarantees?

Paul? Frederic?

Oleg.

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