Messages in this thread | | | Date | Fri, 15 Mar 2013 11:46:44 +0800 | Subject | Re: + atomic-improve-atomic_inc_unless_negative-atomic_dec_unless_positive .patch added to -mm tree | From | Ming Lei <> |
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On Fri, Mar 15, 2013 at 12:24 AM, Oleg Nesterov <oleg@redhat.com> wrote: >> From: Ming Lei <tom.leiming@gmail.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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