> From: Feifei Wang [mailto:feifei.wa...@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.m...@6wind.com/
>
> Suggested-by: Ruifeng Wang <ruifeng.w...@arm.com>
> Signed-off-by: Feifei Wang <feifei.wa...@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.
