Re: [PATCH v3] net/ixgbe: add proper memory barriers for some Rx functions

[Thomas Monjalon <thomas@monjalon.net>]

15/05/2023 04:10, Zhang, Qi Z:
> From: Ruifeng Wang <Ruifeng.Wang@arm.com>
> > From: Min Zhou <zhoumin@loongson.cn>
> > >
> > > Segmentation fault has been observed while running the
> > > ixgbe_recv_pkts_lro() function to receive packets on the Loongson
> > > 3C5000 processor which has 64 cores and 4 NUMA nodes.
> > >
> > > From the ixgbe_recv_pkts_lro() function, we found that as long as the
> > > first packet has the EOP bit set, and the length of this packet is
> > > less than or equal to rxq->crc_len, the segmentation fault will
> > > definitely happen even though on the other platforms. For example, if
> > > we made the first packet which had the EOP bit set had a zero length by
> > force, the segmentation fault would happen on X86.
> > >
> > > Because when processd the first packet the first_seg->next will be
> > > NULL, if at the same time this packet has the EOP bit set and its
> > > length is less than or equal to rxq->crc_len, the following loop will be
> > executed:
> > >
> > >     for (lp = first_seg; lp->next != rxm; lp = lp->next)
> > >         ;
> > >
> > > We know that the first_seg->next will be NULL under this condition. So
> > > the expression of
> > > lp->next->next will cause the segmentation fault.
> > >
> > > Normally, the length of the first packet with EOP bit set will be
> > > greater than rxq-
> > > >crc_len. However, the out-of-order execution of CPU may make the read
> > > >ordering of the
> > > status and the rest of the descriptor fields in this function not be
> > > correct. The related codes are as following:
> > >
> > >         rxdp = &rx_ring[rx_id];
> > >  #1     staterr = rte_le_to_cpu_32(rxdp->wb.upper.status_error);
> > >
> > >         if (!(staterr & IXGBE_RXDADV_STAT_DD))
> > >             break;
> > >
> > >  #2     rxd = *rxdp;
> > >
> > > The sentence #2 may be executed before sentence #1. This action is
> > > likely to make the ready packet zero length. If the packet is the
> > > first packet and has the EOP bit set, the above segmentation fault will
> > happen.
> > >
> > > So, we should add a proper memory barrier to ensure the read ordering
> > > be correct. We also did the same thing in the ixgbe_recv_pkts()
> > > function to make the rxd data be valid even though we did not find
> > segmentation fault in this function.
> > >
> > > Fixes: 8eecb3295ae ("ixgbe: add LRO support")
> > > Cc: stable@dpdk.org
> > >
> > > Signed-off-by: Min Zhou <zhoumin@loongson.cn>
> > > ---
> > > v3:
> > > - Use rte_smp_rmb() as the proper memory barrier instead of rte_rmb()
> > > ---
> > > v2:
> > > - Make the calling of rte_rmb() for all platforms
> > > ---
[...]
> > Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> 
> Applied to dpdk-next-net-intel.
> 
> Thanks
> Qi
> 

Why ignoring checkpatch?
It is saying:
"
Warning in drivers/net/ixgbe/ixgbe_rxtx.c:
Using rte_smp_[r/w]mb
"

Ruifeng proposed "rte_atomic_thread_fence(__ATOMIC_ACQUIRE)"
in a comment on the v2.

I will drop this patch from the pull of next-net-intel branch.