Question/Bug about AF_XDP idx_cq from xsk_ring_cons__peek?

[William Tu <u9012063@xxxxxxxxx>]

Hi,

In my implementation[1], I'm doing s.t like
    tx_done = xsk_ring_cons__peek(&xsk->umem->cq, BATCH_SIZE, &idx_cq);
    if (tx_done > 0) {
        xsk_ring_cons__release(&xsk->umem->cq, tx_done);
        xsk->outstanding_tx -= tx_done;
    }

I expect if tx_done returns 32, then after calling xsk_ring_cons__release,
the next time I call xsk_ring_cons__peek, I should get idx_cq + 32.
However, sometimes I see the same value of idx_cq is returned as previous, even
when tx_done > 0. Is this the expected behavior?

I experiment with xdpsock_user.c with the patch below and run
~/bpf-next/samples/bpf# ./xdpsock -l -N -z -i eth3
using bpf-next commit 5e2ac390fbd08b2a462db66cef2663e4db0d5191

--- a/samples/bpf/xdpsock_user.c
+++ b/samples/bpf/xdpsock_user.c
@@ -444,6 +443,8 @@ static void kick_tx(struct xsk_socket_info *xsk)
        exit_with_error(errno);
 }

+static int prev_idx_cq;
+
 static inline void complete_tx_l2fwd(struct xsk_socket_info *xsk)
 {
        u32 idx_cq = 0, idx_fq = 0;
@@ -463,6 +464,15 @@ static inline void complete_tx_l2fwd(struct
xsk_socket_info *xsk)
                unsigned int i;
                int ret;

+        if (idx_cq == prev_idx_cq) {
+            printf("\n\n ERROR \n\n");
+        }
+        if (idx_cq - prev_idx_cq != rcvd) {
+            printf("\n\n ERROR request %d return %d idx_cq %x prev_cq
%x diff %d\n",
+                   ndescs, rcvd, idx_cq, prev_idx_cq, idx_cq - prev_idx_cq);
+        }
+        prev_idx_cq = idx_cq;
+
                ret = xsk_ring_prod__reserve(&xsk->umem->fq, rcvd, &idx_fq);
                while (ret != rcvd) {
                        if (ret < 0)
@@ -599,7 +609,7 @@ static void tx_only(struct xsk_socket_info *xsk)
        }
 }
Most of the time idx_cq - prev_idx_cq == rcvd, but sometimes I got:
 ERROR request 64 return 64 idx_cq 107f54 prev_cq 107f50 diff 4
 ERROR request 64 return 4 idx_cq 2df794 prev_cq 2df754 diff 64
 ERROR request 60 return 60 idx_cq 2df798 prev_cq 2df794 diff 4
 ERROR request 64 return 64 idx_cq 2df7d4 prev_cq 2df798 diff 60
I wonder if it is a bug?
[1]https://github.com/williamtu/ovs-ebpf/blob/afxdp-v11/lib/netdev-afxdp.c#L719

Thanks
William