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

[Phil Yang <Phil.Yang@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?

Yes. We'd better do that.
I will add a notice for it in this patch. Thanks.

> 
> >
> > 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