WARN_ON in commit 0ffdaf5 "net/sock: add WARN_ON(parent->sk) in sock_graft()"

HI,

With commit 0ffdaf5b41cf4435ece14d1d3e977ce69012a20d "net/sock: add WARN_ON(parent->sk) in sock_graft()",
I can see the WARN_ON is triggered easily by dlm subsystem.

tcp_accept_from_sock() in  fs/dlm/lowcomms.c has the following snippet:

1.    result = sock_create_kern(&init_net, dlm_local_addr[0]->ss_family,
                                      SOCK_STREAM, IPPROTO_TCP, &newsock);
        sk is set by the path "sock_create_kern -> __sock_creat -> pf->create => inet_create -> sock_init_data"
    ...
2.    result = con->sock->ops->accept(con->sock, newsock, O_NONBLOCK, true);
        Then sock_graft is called by "con->sock->ops->accept => inet_accept -> sock_graft "

Does it mean call accept() after the just created socket is not a acceptable behavior? Any comment will be appreciate, thanks.

Regards,
Guoqing