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

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi Thomas,
 
> > The rte_pktmbuf_detach() function should decrease refcnt on a direct
> > buffer.
> >
> > Signed-off-by: Hiroyuki Mikita <h.mikita89@gmail.com>
> > ---
> > v2:
> > * introduced a new function rte_pktmbuf_detach2() which decrease refcnt.
> 
> 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.
Let say right now examples/vhost/main.c has its own pktmbuf_detach_zcp()
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.
Konstantin  

> 
> Please explicit the function name for the detach operation in
> doc/guides/prog_guide/mbuf_lib.rst (whatever detach2 or unattach).
> 
> > * marked rte_pktmbuf_detach() as deprecated.
> > * added comments about refcnt to rte_pktmbuf_attach() and rte_pktmbuf_detach().
> > * checked refcnt when detaching in unit tests.
> > * added this issue to release notes.