[dpdk-dev] [PATCH v3] mbuf: cleanup rte_pktmbuf_lastseg(), fix atomic usage
Hanoch Haim (hhaim)
hhaim at cisco.com
Thu Nov 16 10:06:56 CET 2017
Understood
rte_mbuf_refcnt_update_blind()
should be good., it will take care the RTE_MBUF_REFCNT_ATOMIC
Hanoh
-----Original Message-----
From: Olivier MATZ [mailto:olivier.matz at 6wind.com]
Sent: Thursday, November 16, 2017 10:42 AM
To: Hanoch Haim (hhaim)
Cc: Konstantin Ananyev; Ilya Matveychikov; dev at dpdk.org
Subject: Re: [dpdk-dev] [PATCH v3] mbuf: cleanup rte_pktmbuf_lastseg(), fix atomic usage
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
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