At 2017-08-09 03:45:53, "Cong Wang" <xiyou.wangc...@gmail.com> wrote: >On Mon, Aug 7, 2017 at 6:10 PM, Gao Feng <gfree.w...@vip.163.com> wrote: >> >> Sorry, I don't get you clearly. Why the sock_hold() isn't helpful? > >I already told you, the dereference happends before sock_hold(). > > sock = rcu_dereference(callid_sock[call_id]); > if (sock) { > opt = &sock->proto.pptp; > if (opt->dst_addr.sin_addr.s_addr != s_addr) <=== HERE > sock = NULL; > else > sock_hold(sk_pppox(sock)); > } > >If we don't wait for readers properly, sock could be freed at the >same time when deference it.
Maybe I didn't show my explanation clearly. I think it won't happen as I mentioned in the last email. Because the pptp_release invokes the synchronize_rcu to make sure it, and actually there is no one which would invoke del_chan except pptp_release. It is guaranteed by that the pptp_release doesn't put the sock refcnt until complete all cleanup include marking sk_state as PPPOX_DEAD. In other words, even though the pptp_release is not the last user of this sock, the other one wouldn't invoke del_chan in pptp_sock_destruct. Because the condition "!(sk->sk_state & PPPOX_DEAD)" must be false. As summary, the del_chan and pppox_unbind_sock in pptp_sock_destruct are unnecessary. And it even brings confusing. Best Regards Feng > >> The pptp_release invokes synchronize_rcu after del_chan, it could make sure >> the others has increased the sock refcnt if necessary >> and the lookup is over. >> There is no one could get the sock after synchronize_rcu in pptp_release. > > >If this were true, then this code in pptp_sock_destruct() >would be unneeded: > > if (!(sk->sk_state & PPPOX_DEAD)) { > del_chan(pppox_sk(sk)); > pppox_unbind_sock(sk); > } > > >> >> >> But I think about another problem. >> It seems the pptp_sock_destruct should not invoke del_chan and >> pppox_unbind_sock. >> Because when the sock refcnt is 0, the pptp_release must have be invoked >> already. >> > > >I don't know. Looks like sock_orphan() is only called >in pptp_release(), but I am not sure if there is a case >we call sock destructor before release. > >Also note, this socket is very special, it doesn't support >poll(), sendmsg() or recvmsg()..