Re: [dpdk-dev] [PATCH v3] mbuf: use C11 atomic built-ins for refcnt operations

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>

> >
> > Hi Phil,
> >
> > On Thu, Jul 09, 2020 at 06:10:42PM +0800, Phil Yang wrote:
> > > Use C11 atomic built-ins with explicit ordering instead of
> > > rte_atomic ops which enforce unnecessary barriers on aarch64.
> > >
> > > Signed-off-by: Phil Yang <phil.yang@arm.com>
> > > Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> > > ---
> > > v3:
> > > 1.Fix ABI breakage.
> > > 2.Simplify data type cast.
> > >
> > > v2:
> > > Fix ABI issue: revert the rte_mbuf_ext_shared_info struct refcnt
> > > field to refcnt_atomic.
> > >
> > >  lib/librte_mbuf/rte_mbuf.c      |  1 -
> > >  lib/librte_mbuf/rte_mbuf.h      | 19 ++++++++++---------
> > >  lib/librte_mbuf/rte_mbuf_core.h |  2 +-
> > >  3 files changed, 11 insertions(+), 11 deletions(-)
> > >
> <snip>
> > >
> > >  /* Reinitialize counter before mbuf freeing. */ diff --git
> > > a/lib/librte_mbuf/rte_mbuf_core.h
> > b/lib/librte_mbuf/rte_mbuf_core.h
> > > index 16600f1..d65d1c8 100644
> > > --- a/lib/librte_mbuf/rte_mbuf_core.h
> > > +++ b/lib/librte_mbuf/rte_mbuf_core.h
> > > @@ -679,7 +679,7 @@ typedef void
> > (*rte_mbuf_extbuf_free_callback_t)(void *addr, void *opaque);
> > >  struct rte_mbuf_ext_shared_info {
> > >  rte_mbuf_extbuf_free_callback_t free_cb; /**< Free callback
> > function */
> > >  void *fcb_opaque;                        /**< Free callback argument */
> > > -rte_atomic16_t refcnt_atomic;        /**< Atomically accessed refcnt */
> > > +uint16_t refcnt_atomic;              /**< Atomically accessed refcnt */
> > >  };
> >
> > To avoid an API breakage (i.e. currently, an application that accesses
> > to refcnt_atomic expects that its type is rte_atomic16_t), I suggest
> > to do the same than in the mbuf struct:
> >
> > union {
> > rte_atomic16_t refcnt_atomic;
> > uint16_t refcnt;
> > };
> >
> > I hope the ABI checker won't complain.
> >
> > It will also be better for 20.11 when the deprecated fields will be
> > renamed: the remaining one will be called 'refcnt' in both mbuf and
> > mbuf_ext_shared_info.
Does this need a deprecation notice in 20.08?

> 
> Got it. I agree with you.
> It should work. In my local test machine, the ABI checker happy with this
> approach.
> Once the test is done, I will upstream the new patch.
> 
> Appreciate your comments.
> 
> Thanks,
> Phil