Messages in this thread | | | Date | Mon, 11 Mar 2013 16:45:09 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH] atomic: improve atomic_inc_unless_negative/atomic_dec_unless_positive |
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On Sat, 9 Mar 2013 23:38:21 +0800 Ming Lei <tom.leiming@gmail.com> 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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