Re: [dpdk-stable] [dpdk-dev] [PATCH v3 2/2] net/i40e: replace SMP barrier with thread fence

[Lance Richardson <lance.richardson@broadcom.com>]

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On Thu, Jul 8, 2021 at 8:09 AM Zhang, Qi Z <qi.z.zhang@intel.com> wrote:
>
>
>
> > -----Original Message-----
> > From: Joyce Kong <joyce.kong@arm.com>
> > Sent: Tuesday, July 6, 2021 2:54 PM
> > To: Xing, Beilei <beilei.xing@intel.com>; Zhang, Qi Z <qi.z.zhang@intel.com>;
> > ruifeng.wang@arm.com; honnappa.nagarahalli@arm.com; Richardson, Bruce
> > <bruce.richardson@intel.com>; Zhang, Helin <helin.zhang@intel.com>
> > Cc: dev@dpdk.org; stable@dpdk.org; nd@arm.com
> > Subject: [PATCH v3 2/2] net/i40e: replace SMP barrier with thread fence
> >
> > Simply replace the SMP barrier with atomic thread fence for i40e hw ring sacn,
> > if there is no synchronization point.
> >
> > Signed-off-by: Joyce Kong <joyce.kong@arm.com>
> > Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> > ---
> >  drivers/net/i40e/i40e_rxtx.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/i40e/i40e_rxtx.c b/drivers/net/i40e/i40e_rxtx.c index
> > 9aaabfd92..86e2f083e 100644
> > --- a/drivers/net/i40e/i40e_rxtx.c
> > +++ b/drivers/net/i40e/i40e_rxtx.c
> > @@ -482,7 +482,8 @@ i40e_rx_scan_hw_ring(struct i40e_rx_queue *rxq)
> >                                       I40E_RXD_QW1_STATUS_SHIFT;
> >               }
> >
> > -             rte_smp_rmb();
> > +             /* This barrier is to order loads of different words in the descriptor */
> > +             rte_atomic_thread_fence(__ATOMIC_ACQUIRE);
>
> Now for x86, you actually replace a compiler barrier with a memory fence, this may have potential performance impact which need additional resource to investigate

No memory fence instruction is generated for
__ATOMIC_ACQUIRE on x86 for any version of gcc
or clang that I've tried, based on experiments here:

    https://godbolt.org/z/Yxr1vGhKP