On Fri, Mar 15, 2013 at 12:24 AM, Oleg Nesterov <[email protected]> wrote:
>> From: Ming Lei <[email protected]>
>> 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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