On Sat, 9 Mar 2013 23:38:21 +0800 Ming Lei <[email protected]> wrote:
> 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.
>
> Considered memory barrier, bus lock, cache walking, etc. things may be
> involved in atomic_cmpxchg(), it is much expensive than atomic_read(),
> which is just the simple below:
>
> (*(volatile int *)&(v)->counter)
>
> so this patch can save one extra atomic_cmpxchg() for the two
> helpers under general situation, and should improve them a bit.
>
> ...
>
> --- a/include/linux/atomic.h
> +++ b/include/linux/atomic.h
> @@ -63,26 +63,34 @@ static inline int atomic_inc_not_zero_hint(atomic_t *v,
> int hint)
> #ifndef atomic_inc_unless_negative
> static inline int atomic_inc_unless_negative(atomic_t *p)
> {
> - int v, v1;
> - for (v = 0; v >= 0; v = v1) {
> - v1 = atomic_cmpxchg(p, v, v + 1);
> - if (likely(v1 == v))
> + int v, t;
> +
> + v = atomic_read(p);
> + while (1) {
> + if (unlikely(v < 0))
> + return 0;
> + t = atomic_cmpxchg(p, v, v + 1);
> + if (likely(t == v))
> return 1;
> + v = t;
> }
> - return 0;
It looks right to me...
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