Re: Could DPDK ring provide peek interface or any comments for this function?

[Olivier MATZ <olivier.matz@6wind.com>]

Hi,


On 08/04/2015 04:20 AM, Yan, Liming (Nokia - CN/Hangzhou) wrote:
> Hi,
>     As we see, DPDK ring has mainly the enqueue/dequeue APIs for SINGLE/MULTI producer/consumer.  I have a requirement in work to take a peek on the top object in the ring. I don't want to consume it, just check the reference of the object.  How could this be supported?   Or any alternative solution for my requirement?  Thanks.
>     I wrote my own simple peek interface but it's not MC-safe. I think it's better if DPDK can support it officially.

I think it's a good idea. I think we could avoid a test by using
the same naming convention than other functions applying to single
consumer rings.

I'm not sure the function is safe on mc rings: the pointer returned
via peek() should not be accessed while another core dequeues the
object and modify/free it.


A simplified version would be something like that:


/**
  * Peek one object from a single-consumer ring
  *
  * The returned object is not dequeued. This function does not work
  * on a multi-consumers ring.
  *
  * @param r
  *   A pointer to the ring structure.
  * @param obj_p
  *   A pointer to a void * pointer (object) that will be filled.
  * @return
  *   - 0: Success; objects dequeued.
  *   - -ENOENT: Not enough entries in the ring to peek
  */
static inline int
rte_ring_sc_peek(struct rte_ring *r, void **obj_p)
{
	if (unlikely(rte_ring_empty(r)))
		return -ENOENT;
	*obj_p = r->ring[r->cons.head & r->prod.mask];
	return 0;
}


Regards,
Olivier


>
>
>   /**
> + * Just take a peek on the top object from a ring but not consume it.
> + * Note: This interface is MC and multi-thread not safe.
> + *       It can only be used for ring with RING_F_SC_DEQ attribute.
> + *
> + * @param r
> + *   A pointer to the ring structure.
> + * @param obj_p
> + *   A pointer to a void * pointer (object) that will be filled.
> + * @return
> + *   - 0: Success, object is peeked.
> + *   - -ENOENT: Not entries in the ring.
> + *   - - EPERM: Operation not permitted/supported
> + */
> +static inline int __attribute__((always_inline))
> +rte_ring_peek(const struct rte_ring *r, void **obj_p)
> +{
> +	uint32_t cons_head, prod_tail;
> +
> +	if (r->cons.sc_dequeue) {
> +		cons_head = r->cons.head;
> +		prod_tail = r->prod.tail;
> +
> +		if (prod_tail - cons_head == 0) {
> +			return -ENOENT;
> +		}
> +		*obj_p = r->ring[cons_head & r->prod.mask];
> +	} else {
> +		return -EPERM;
> +	}
> +	return 0;
> +}
> +
> +/**
>
>
> --------^_^--------
> Best Regards
> Yan Limin
>
>