Can you tell me the output of /proc/sys/net/core/netdev_tstamp_prequeue on
your machine?
/proc/sys/net/core/netdev_tstamp_prequeue is set to 1 (unmodified, default)
I tried to dig a little deeper in timestamping:
1. (net/core/dev.c) I found that static_key_false(&netstamp_needed) is always 0, resulting that the timestamp is never set by net_timestamp_check in netif_receive_skb_internal.
2. (net/core/dev.c) static_key_false(&netstamp_needed) is 0 because net_enable_timestamp is never called.
3. (net/core/sock.c) net_enable_timestamp is never called because SK_FLAGS_TIMESTAMP is not set
4. (net/core/sock.c) SK_FLAGS_TIMESTAMP is not set because neither of SOCK_TIMESTAMP or SOCK_TIMESTAMPING_RX_SOFTWARE is set
5. (net/core/sock.c) SOCK_TIMESTAMP or SOCK_TIMESTAMPING_RX_SOFTWARE is not set because timestamping is an optional feature (according to http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/networking/timestamping.txt?id=b953c0d234bc72e8489d3bf51a276c5c4ec85345) not enabled in my use case (even if netdev_tstamp_prequeue is set to 1)
So the original assumption for the was correct: The correctness of the skb equality check depends on a feature that is not enabled by default (respectively user configurable).
Do you agree with this?