Re: [PATCH] atomic: improveatomic_inc_unless_negative/atomic_dec_unless_positive

[Andrew Morton]

On Sat,  9 Mar 2013 23:38:21 +0800 Ming Lei <tom.leiming@xxxxxxxxx> 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...
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/