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Message-ID: <b4470cec-7b9b-5ce5-01e0-9270f6564fbb@linux.ibm.com> Date: Thu, 5 Oct 2023 20:14:37 +0200 From: Wenjia Zhang <wenjia@...ux.ibm.com> To: "D. Wythe" <alibuda@...ux.alibaba.com>, Alexandra Winter <wintera@...ux.ibm.com> Cc: jaka@...ux.ibm.com, kgraul@...ux.ibm.com, kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org, linux-s390@...r.kernel.org, linux-rdma@...r.kernel.org Subject: Re: [PATCH net] net/smc: fix panic smc_tcp_syn_recv_sock() while closing listen socket On 26.09.23 11:06, D. Wythe wrote: > > > On 9/26/23 3:18 PM, Alexandra Winter wrote: >> >> On 26.09.23 05:00, D. Wythe wrote: >>> You are right. The key point is how to ensure the valid of smc sock >>> during the life time of clc sock, If so, READ_ONCE is good >>> enough. Unfortunately, I found that there are no such guarantee, so >>> it's still a life-time problem. >> Did you discover a scenario, where clc sock could live longer than smc >> sock? >> Wouldn't that be a dangerous scenario in itself? I still have some >> hope that the lifetime of an smc socket is by design longer >> than that of the corresponding tcp socket. > > > Hi Alexandra, > > Yes there is. Considering scenario: > > tcp_v4_rcv(skb) > > /* req sock */ > reqsk = _inet_lookup_skb(skb) > > /* listen sock */ > sk = reqsk(reqsk)->rsk_listener; > sock_hold(sk); > tcp_check_req(sk) > > > smc_release /* release > smc listen sock */ > __smc_release > smc_close_active() /* smc_sk->sk_state = SMC_CLOSED; */ > if > (smc_sk->sk_state == SMC_CLOSED) > smc_clcsock_release(); > sock_release(clcsk); /* close clcsock */ > sock_put(sk); /* might not the final refcnt */ > > sock_put(smc_sk) /* might be the final refcnt of smc_sock */ > > syn_recv_sock(sk...) > /* might be the final refcnt of tcp listen sock */ > sock_put(sk); > > Fortunately, this scenario only affects smc_syn_recv_sock and > smc_hs_congested, as other callbacks already have locks to protect smc, > which can guarantee that the sk_user_data is either NULL (set in > smc_close_active) or valid under the lock. > I'm kind of confused with this scenario. How could the smc_clcsock_release()->sock_release(clcsk) happen? Because the syn_recv_sock happens short prior to accept(), that means that the &smc->tcp_listen_work is already triggered but the real accept() is still not happening. At this moment, the incoming connection is being added into the accept queue. Thus, if the sk->sk_state is changed from SMC_LISTEN to SMC_CLOSED in smc_close_active(), there is still "flush_work(&smc->tcp_listen_work);" after that. That ensures the smc_clcsock_release() should not happen, if smc_clcsock_accept() is not finished. Do you think that the execution of the &smc->tcp_listen_work is already done? Or am I missing something? >> Considering the const, maybe >>> we need to do : >>> >>> 1. hold a refcnt of smc_sock for syn_recv_sock to keep smc sock valid >>> during life time of clc sock >>> 2. put the refcnt of smc_sock in sk_destruct in tcp_sock to release >>> the very smc sock . >>> >>> In that way, we can always make sure the valid of smc sock during the >>> life time of clc sock. Then we can use READ_ONCE rather >>> than lock. What do you think ? >> I am not sure I fully understand the details what you propose to do. >> And it is not only syn_recv_sock(), right? >> You need to consider all relations between smc socks and tcp socks; >> fallback to tcp, initial creation, children of listen sockets, >> variants of shutdown, ... Preferrably a single simple mechanism covers >> all situations. Maybe there is such a mechanism already today? >> (I don't think clcsock->sk->sk_user_data or sk_callback_lock provide >> this general coverage) >> If we really have a gap, a general refcnt'ing on smc sock could be a >> solution, but needs to be designed carefully. > > You are right , we need designed it with care, we will try the > referenced solutions internally first, and I will also send some RFCs so > that everyone can track the latest progress > and make it can be all agreed. >> Many thanks to you and the team to help make smc more stable and robust. > > Our pleasure 😁. The stability of smc is important to us too. > > Best wishes, > D. Wythe > >
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