2013/3/9 Ming Lei <[email protected]>: > 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. > > Cc: Andrew Morton <[email protected]> > Cc: Shaohua Li <[email protected]> > Cc: Al Viro <[email protected]> > Signed-off-by: Ming Lei <[email protected]> > --- > include/linux/atomic.h | 28 ++++++++++++++++++---------- > 1 file changed, 18 insertions(+), 10 deletions(-) > > diff --git a/include/linux/atomic.h b/include/linux/atomic.h > index 5b08a85..aa951d8 100644 > --- 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;
But atomic_read() lacks the full memory barrier that is needed for proper atomicity here. For example if the initial value of p is -1 and another CPU just did an atomic_inc() that resulted in the new value to be 0, the above atomic_read() might return -1 because there is no guarantee it's seeing the recent update on the remote CPU. atomic_cmpxchg() OTOH provides that guarantee. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
