[dpdk-dev] [PATCH v2] mbuf: decrease refcnt when detaching

[Hiroyuki MIKITA]

I think this behavior is not part of the API, it is a bug.

I agree that detach() frees the direct mbuf when refcnt becomes 0,
Konstantin suggests.
It is a right behavior of reference counting.

Regards,
Hiroyuki

2016-05-18 0:45 GMT+09:00 Ananyev, Konstantin <konstantin.ananyev at intel.com>:
>
>
>> -----Original Message-----
>> From: Thomas Monjalon [mailto:thomas.monjalon at 6wind.com]
>> Sent: Tuesday, May 17, 2016 3:19 PM
>> To: Ananyev, Konstantin
>> Cc: dev at dpdk.org; Hiroyuki Mikita; olivier.matz at 6wind.com
>> Subject: Re: [dpdk-dev] [PATCH v2] mbuf: decrease refcnt when detaching
>>
>> 2016-05-17 13:44, Ananyev, Konstantin:
>> > From: Thomas Monjalon [mailto:thomas.monjalon at 6wind.com]
>> > > 2016-05-17 12:59, Ananyev, Konstantin:
>> > > > > > The rte_pktmbuf_detach() function should decrease refcnt on a direct
>> > > > > > buffer.
>> > > > >
>> > > > > As you have noticed, "whenever the indirect buffer is detached,
>> > > > > the reference counter on the direct buffer is decremented."
>> > > > > So the current behaviour of rte_pktmbuf_detach() is buggy.
>> > > > > Why not fix it without renaming?
>> > > > > If you consider this behavioral bug is part of the API, we
>> > > > > can fix it in a new function unattach and deprecate detach.
>> > > > > But Konstantin, why do you want to keep a restore function?
>> > > > > What is the need?
>> > > >
>> > > > I think it might be a useful functionality in some situations:
>> > > > some users can attach/detach to external memory buffers (no mbufs)
>> > > > and similar functionality is required.
>> > >
>> > > Attach to external memory buffer (raw buffer) is not currently supported.
>> > >
>> > > > Let say right now examples/vhost/main.c has its own pktmbuf_detach_zcp()
>> > >
>> > > You should look at the commit http://dpdk.org/commit/68363d85
>> > >   "examples/vhost: remove the non-working zero copy code"
>> > >
>> > > > which is doing pretty much the same - restore original values, after detaching
>> > > > mbuf from external (virtio) memory buffer.
>> > > > Would be good if we'll use a standard API function here.
>> > >
>> > > You are welcome to implement mbuf attach to raw buffer.
>> > > But it is not a requirement for this fix.
>> >
>> > Hmm, still not sure why we can't keep an existing function?
>>
>> Because it does not do what its name (and doc) suggest.
>>
>> > Obviously it wouldn't cost anything and I still think might be useful.
>>
>> It costs to overcomplicate API for only a half support.
>
> I still think it is better to have it then not, but wouldn't insist here.
> Konstantin
>
>> If you need the feature "attach to raw", please implement it completely.
>