[dpdk-dev] [PATCH v3] mbuf: cleanup rte_pktmbuf_lastseg(), fix atomic usage

[Olivier MATZ]

Hi Hanoh,

On Thu, Nov 16, 2017 at 07:16:31AM +0000, Hanoch Haim (hhaim) wrote:
> Hi Oliver, 
> 
> It's hard for me to follow this thread. 

Yes, here are some few tips to make it easier to follow:
- avoid top-posting
- prefix quoted lines with "> "
- describe the problem and how you solve it in the commit log
- one problem = one patch

> 1)  It is not about clear/not-clear, it is error prone to *replicate* code that has the same logic.
> 
> "I'm not convinced that:
> 
>     __rte_pktmbuf_reset_nb_segs(m);
> 
> is clearer than:
> 
>    m->next = NULL;
>    m->nb_segs = 1;
> 
> Anyway, I agree this should not be part of this patch. We should only keep the fix.
> "

rte_mbuf_refcnt_update() was not used in rte_pktmbuf_prefree_seg() to
avoid reading the refcount twice.

The problem of having clear or unclear is fundamental. I don't see the point of
having a function __rte_pktmbuf_reset_nb_segs(). Keeping the two affectations
makes things explicit.

> 2) This definitely does not look good. 
> All the point in my patch is to move the ref-cnt operations to set of API that already taking care of RTE_MBUF_REFCNT_ATOMIC
> 
> +               /* We don't use rte_mbuf_refcnt_update() because we already
> +                * tested that refcnt != 1.
> +                */
> +#ifdef RTE_MBUF_REFCNT_ATOMIC
> +               ret = rte_atomic16_add_return(&m->refcnt_atomic, -1); 
> +#else
> +               ret = --m->refcnt;
> +#endif
> +               if (ret != 0)
> +                       return NULL;
> 

We cannot use the existing API taking care of atomic refcount
rte_mbuf_refcnt_update() because it would read the refcount twice.

We cannot change the behavior of rte_mbuf_refcnt_update() because it's a
public API.

An option proposed by Konstantin is to introduce a new helper
rte_mbuf_refcnt_update_blind() that does the same than
rte_mbuf_refcnt_update() but without the first test.  It think it is a
bit overkill to have this function for one caller.

That's why I end up with this patch. I don't see why it would be an
issue to have a mbuf ifdef inside the mbuf code.

Olivier