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

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Date:	Fri, 15 Mar 2013 11:46:44 +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 12:24 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>> From: Ming Lei <tom.leiming@...il.com>
>> Subject: atomic: improve atomic_inc_unless_negative/atomic_dec_unless_positive
>>
>> Generally, both atomic_inc_unless_negative() and
>> atomic_dec_unless_positive() need at least two atomic_cmpxchg() to
>> complete the atomic operation.  In fact, the 1st atomic_cmpxchg() is just
>> used to read current value of the atomic variable at most times.
>
> Agreed, this looks ugly...
>
> But can't we make a simpler patch and keep the code simple/readable ?
>
> Oleg.
>
> --- x/include/linux/atomic.h
> +++ x/include/linux/atomic.h
> @@ -64,7 +64,7 @@ static inline int atomic_inc_not_zero_hi
>  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.

>                 if (likely(v1 == v))
>                         return 1;
> @@ -77,7 +77,7 @@ static inline int atomic_inc_unless_nega
>  static inline int atomic_dec_unless_positive(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);

Similar with above.

>                 if (likely(v1 == v))
>                         return 1;
>


Thanks,
-- 
Ming Lei
--
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