Hi Hiroyuki,
On 05/15/2016 05:50 PM, Hiroyuki Mikita wrote:
> The rte_pktmbuf_detach() function should decrease refcnt on a direct
> buffer.
>
> Signed-off-by: Hiroyuki Mikita <h.mikita89 at gmail.com>
> ---
> lib/librte_mbuf/rte_mbuf.h | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> index 529debb..3b117ca 100644
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -1468,9 +1468,11 @@ static inline void rte_pktmbuf_attach(struct rte_mbuf
> *mi, struct rte_mbuf *m)
> */
> static inline void rte_pktmbuf_detach(struct rte_mbuf *m)
> {
> + struct rte_mbuf *md = rte_mbuf_from_indirect(m);
> struct rte_mempool *mp = m->pool;
> uint32_t mbuf_size, buf_len, priv_size;
>
> + rte_mbuf_refcnt_update(md, -1);
> priv_size = rte_pktmbuf_priv_size(mp);
> mbuf_size = sizeof(struct rte_mbuf) + priv_size;
> buf_len = rte_pktmbuf_data_room_size(mp);
> @@ -1498,7 +1500,7 @@ __rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
> if (RTE_MBUF_INDIRECT(m)) {
> struct rte_mbuf *md = rte_mbuf_from_indirect(m);
> rte_pktmbuf_detach(m);
> - if (rte_mbuf_refcnt_update(md, -1) == 0)
> + if (rte_mbuf_refcnt_read(md) == 0)
> __rte_mbuf_raw_free(md);
> }
> return m;
>
Thanks for submitting this patch. I agree that rte_pktmbuf_attach()
and rte_pktmbuf_detach() are not symmetrical, but I think your patch
could trigger some race conditions.
Example:
- init: m, c1 and c2 are direct mbuf
- rte_pktmbuf_attach(c1, m); # c1 becomes a clone of m
- rte_pktmbuf_attach(c2, m); # c2 becomes another clone of m
- rte_pktmbuf_free(m);
- after that, we have:
- m is a direct mbuf with refcnt = 2
- c1 is an indirect mbuf pointing to data of m
- c2 is an indirect mbuf pointing to data of m
- if we call rte_pktmbuf_free(c1) and rte_pktmbuf_free(c2) on 2
different cores at the same time, m can be freed twice because
(rte_mbuf_refcnt_read(md) == 0) can be true on both cores.
I think the proper way of doing would be to have rte_pktmbuf_detach()
returning the value of rte_mbuf_refcnt_update(md, -1), ensuring that
only one core will call _rte_mbuf_raw_free().
In the unit tests, in test_attach_from_different_pool(), the mbuf m
is never freed due to this behavior. That shows the current api is
a bit misleading. I think it should also be fixed in the patch.
Another issue is that it will break the API.
To avoid issues in applications relying on the current behavior of
rte_pktmbuf_detach(), I'd say we should keep the function as-is and
mark it as deprecated. Then, introduce a new function
rte_pktmbuf_detach2() (or any better name :) ) that changes the
behavior to what you suggest. An entry in the release note would
also be helpful.
The other option is to let things as-is and just document the behavior
of rte_pktmbuf_detach(), explicitly saying that it is not symmetrical
with the attach. But I'd prefer the first option.
Thoughts ?
Regards,
Olivier
