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

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2016-05-17 13:44, Ananyev, Konstantin:
> From: Thomas Monjalon [mailto:thomas.monjalon@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.
If you need the feature "attach to raw", please implement it completely.