[sheepdog] [PATCH v2 2/4] util: introduce a new data type refcnt_t for reference counting

[Liu Yuan]

On Tue, Jul 09, 2013 at 04:42:01PM +0900, Hitoshi Mitake wrote:
> From: Hitoshi Mitake <mitake.hitoshi at gmail.com>
> 
> Using raw int for reference counting directly is dangerous because it
> cannot detect ++ or -- for these counters at compilation time. So this
> patch implements a new data type refcnt_t for safe reference counting
> which can be done by multiple threads in parallel.
> 
> Signed-off-by: Hitoshi Mitake <mitake.hitoshi at lab.ntt.co.jp>
> ---
>  include/util.h |   36 ++++++++++++++++++++++++++++++++++++
>  1 file changed, 36 insertions(+)
> 
> diff --git a/include/util.h b/include/util.h
> index 1517680..7e60ccc 100644
> --- a/include/util.h
> +++ b/include/util.h
> @@ -212,6 +212,42 @@ static inline void uatomic_set_false(uatomic_bool *val)
>  	(typeof(*(p)))ret;				\
>  })
>  
> +/*
> + * refcnt_t: reference counter which can be manipulated by multiple threads
> + * safely
> + */
> +
> +typedef struct {
> +	int val;
> +} refcnt_t;
> +
> +static inline int refcnt_read(refcnt_t *rc)
> +{
> +	return rc->val;
> +}
> +
> +static inline void refcnt_set(refcnt_t *rc, int val)
> +{
> +	rc->val = val;
> +}
> +
> +static inline void refcnt_inc(refcnt_t *rc)
> +{
> +	uatomic_inc(&rc->val);
> +}
> +
> +static inline void refcnt_dec(refcnt_t *rc)
> +{
> +	uatomic_dec(&rc->val);
> +	assert(0 <= rc->val);
> +}
> +
> +static inline int refcnt_dec_return(refcnt_t *rc)
> +{
> +	refcnt_dec(rc);
> +	return refcnt_read(rc);
> +}

I don't think this refcnt_dec_return is atomic operatoin. You should rely on the
atomic_sub_return()...anyway, I am not yet convinced to use wrapper instead of
atomic helpers.

Thanks
Yuan