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

[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
---
 drivers/net/ixgbe/ixgbe_rxtx.c | 39 ++++++++++++----------------------
 1 file changed, 13 insertions(+), 26 deletions(-)

diff --git a/drivers/net/ixgbe/ixgbe_rxtx.c b/drivers/net/ixgbe/ixgbe_rxtx.c
index 6b3d3a4d1a..80bcaef093 100644
--- a/drivers/net/ixgbe/ixgbe_rxtx.c
+++ b/drivers/net/ixgbe/ixgbe_rxtx.c
@@ -1823,6 +1823,12 @@ ixgbe_recv_pkts(void *rx_queue, struct rte_mbuf **rx_pkts,
 		staterr = rxdp->wb.upper.status_error;
 		if (!(staterr & rte_cpu_to_le_32(IXGBE_RXDADV_STAT_DD)))
 			break;
+
+		/*
+		 * This barrier is to ensure that status_error which includes DD
+		 * bit is loaded before loading of other descriptor words.
+		 */
+		rte_smp_rmb();
 		rxd = *rxdp;
 
 		/*
@@ -2089,32 +2095,8 @@ ixgbe_recv_pkts_lro(void *rx_queue, struct rte_mbuf **rx_pkts, uint16_t nb_pkts,
 
 next_desc:
 		/*
-		 * The code in this whole file uses the volatile pointer to
-		 * ensure the read ordering of the status and the rest of the
-		 * descriptor fields (on the compiler level only!!!). This is so
-		 * UGLY - why not to just use the compiler barrier instead? DPDK
-		 * even has the rte_compiler_barrier() for that.
-		 *
-		 * But most importantly this is just wrong because this doesn't
-		 * ensure memory ordering in a general case at all. For
-		 * instance, DPDK is supposed to work on Power CPUs where
-		 * compiler barrier may just not be enough!
-		 *
-		 * I tried to write only this function properly to have a
-		 * starting point (as a part of an LRO/RSC series) but the
-		 * compiler cursed at me when I tried to cast away the
-		 * "volatile" from rx_ring (yes, it's volatile too!!!). So, I'm
-		 * keeping it the way it is for now.
-		 *
-		 * The code in this file is broken in so many other places and
-		 * will just not work on a big endian CPU anyway therefore the
-		 * lines below will have to be revisited together with the rest
-		 * of the ixgbe PMD.
-		 *
-		 * TODO:
-		 *    - Get rid of "volatile" and let the compiler do its job.
-		 *    - Use the proper memory barrier (rte_rmb()) to ensure the
-		 *      memory ordering below.
+		 * It is necessary to use a proper memory barrier to ensure the
+		 * memory ordering below.
 		 */
 		rxdp = &rx_ring[rx_id];
 		staterr = rte_le_to_cpu_32(rxdp->wb.upper.status_error);
@@ -2122,6 +2104,11 @@ ixgbe_recv_pkts_lro(void *rx_queue, struct rte_mbuf **rx_pkts, uint16_t nb_pkts,
 		if (!(staterr & IXGBE_RXDADV_STAT_DD))
 			break;
 
+		/*
+		 * This barrier is to ensure that status_error which includes DD
+		 * bit is loaded before loading of other descriptor words.
+		 */
+		rte_smp_rmb();
 		rxd = *rxdp;
 
 		PMD_RX_LOG(DEBUG, "port_id=%u queue_id=%u rx_id=%u "
-- 
2.31.1