[PATCH v1] mbuf: remove the redundant code for mbuf prefree

[Morten Brørup]

> From: Feifei Wang [mailto:feifei.wang2 at arm.com]
> Sent: Monday, 4 December 2023 03.39
> 
> For 'rte_pktmbuf_prefree_seg' function, 'rte_mbuf_refcnt_read(m) == 1'
> and '__rte_mbuf_refcnt_update(m, -1) == 0' are the same cases where
> mbuf's refcnt value should be 1. Thus we can simplify the code and
> remove the redundant part.
> 
> Furthermore, according to [1], when the mbuf is stored inside the
> mempool, the m->refcnt value should be 1. And then it is detached
> from its parent for an indirect mbuf. Thus change the description of
> 'rte_pktmbuf_prefree_seg' function.
> 
> [1] https://patches.dpdk.org/project/dpdk/patch/20170404162807.20157-4-
> olivier.matz at 6wind.com/
> 
> Suggested-by: Ruifeng Wang <ruifeng.wang at arm.com>
> Signed-off-by: Feifei Wang <feifei.wang2 at arm.com>
> ---
>  lib/mbuf/rte_mbuf.h | 22 +++-------------------
>  1 file changed, 3 insertions(+), 19 deletions(-)
> 
> diff --git a/lib/mbuf/rte_mbuf.h b/lib/mbuf/rte_mbuf.h
> index 286b32b788..42e9b50d51 100644
> --- a/lib/mbuf/rte_mbuf.h
> +++ b/lib/mbuf/rte_mbuf.h
> @@ -1328,7 +1328,7 @@ static inline int
> __rte_pktmbuf_pinned_extbuf_decref(struct rte_mbuf *m)
>   *
>   * This function does the same than a free, except that it does not
>   * return the segment to its pool.
> - * It decreases the reference counter, and if it reaches 0, it is
> + * It decreases the reference counter, and if it reaches 1, it is

No, the original description is correct.
However, the reference counter is set to 1 when put back in the pool, as a shortcut so it isn't needed to be set back to 1 when allocated from the pool.

>   * detached from its parent for an indirect mbuf.
>   *
>   * @param m
> @@ -1358,25 +1358,9 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)

The preceding "if (likely(rte_mbuf_refcnt_read(m) == 1)) {" is only a shortcut for the likely case.

>  			m->nb_segs = 1;
> 
>  		return m;
> -
> -	} else if (__rte_mbuf_refcnt_update(m, -1) == 0) {
> -
> -		if (!RTE_MBUF_DIRECT(m)) {
> -			rte_pktmbuf_detach(m);
> -			if (RTE_MBUF_HAS_EXTBUF(m) &&
> -			    RTE_MBUF_HAS_PINNED_EXTBUF(m) &&
> -			    __rte_pktmbuf_pinned_extbuf_decref(m))
> -				return NULL;
> -		}
> -
> -		if (m->next != NULL)
> -			m->next = NULL;
> -		if (m->nb_segs != 1)
> -			m->nb_segs = 1;
> -		rte_mbuf_refcnt_set(m, 1);
> -
> -		return m;
>  	}
> +
> +	__rte_mbuf_refcnt_update(m, -1);
>  	return NULL;
>  }
> 
> --
> 2.25.1

NAK.

This patch is not race safe. With the patch:

This thread:
if (likely(rte_mbuf_refcnt_read(m) == 1)) { // Assume it's 2.

The other thread:
if (likely(rte_mbuf_refcnt_read(m) == 1)) { // It's 2.
__rte_mbuf_refcnt_update(m, -1); // Now it's 1.
return NULL;

This thread:
__rte_mbuf_refcnt_update(m, -1); // Now it's 0.
return NULL;

None of the threads have done the "prefree" work.