Date: Thu, 19 Oct 2023 13:54:58 +0200
From: Wenjia Zhang <wenjia@...ux.ibm.com>
To: "D. Wythe" <alibuda@...ux.alibaba.com>, dust.li@...ux.alibaba.com,
        kgraul@...ux.ibm.com, jaka@...ux.ibm.com, wintera@...ux.ibm.com
Cc: kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-rdma@...r.kernel.org
Subject: Re: [PATCH net 1/5] net/smc: fix dangling sock under state
 SMC_APPFINCLOSEWAIT



On 19.10.23 10:09, D. Wythe wrote:
> 
> 
> On 10/18/23 1:03 AM, Wenjia Zhang wrote:
>>
>>
>> On 17.10.23 04:00, D. Wythe wrote:
>>>
>>>
>>> On 10/13/23 8:27 PM, Dust Li wrote:
>>>> On Fri, Oct 13, 2023 at 01:52:09PM +0200, Wenjia Zhang wrote:
>>>>>
>>>>> On 13.10.23 07:32, Dust Li wrote:
>>>>>> On Thu, Oct 12, 2023 at 01:51:54PM +0200, Wenjia Zhang wrote:
>>>>>>>
>>>>>>> On 12.10.23 04:37, D. Wythe wrote:
>>>>>>>>
>>>>>>>> On 10/12/23 4:31 AM, Wenjia Zhang wrote:
>>>>>>>>>
>>>>>>>>> On 11.10.23 09:33, D. Wythe wrote:
>>>>>>>>>> From: "D. Wythe" <alibuda@...ux.alibaba.com>
>>>>>>>>>>
>>>>>>>>>> Considering scenario:
>>>>>>>>>>
>>>>>>>>>>                   smc_cdc_rx_handler_rwwi
>>>>>>>>>> __smc_release
>>>>>>>>>>                   sock_set_flag
>>>>>>>>>> smc_close_active()
>>>>>>>>>> sock_set_flag
>>>>>>>>>>
>>>>>>>>>> __set_bit(DEAD)            __set_bit(DONE)
>>>>>>>>>>
>>>>>>>>>> Dues to __set_bit is not atomic, the DEAD or DONE might be lost.
>>>>>>>>>> if the DEAD flag lost, the state SMC_CLOSED  will be never be 
>>>>>>>>>> reached
>>>>>>>>>> in smc_close_passive_work:
>>>>>>>>>>
>>>>>>>>>> if (sock_flag(sk, SOCK_DEAD) &&
>>>>>>>>>>       smc_close_sent_any_close(conn)) {
>>>>>>>>>>       sk->sk_state = SMC_CLOSED;
>>>>>>>>>> } else {
>>>>>>>>>>       /* just shutdown, but not yet closed locally */
>>>>>>>>>>       sk->sk_state = SMC_APPFINCLOSEWAIT;
>>>>>>>>>> }
>>>>>>>>>>
>>>>>>>>>> Replace sock_set_flags or __set_bit to set_bit will fix this 
>>>>>>>>>> problem.
>>>>>>>>>> Since set_bit is atomic.
>>>>>>>>>>
>>>>>>>>> I didn't really understand the scenario. What is
>>>>>>>>> smc_cdc_rx_handler_rwwi()? What does it do? Don't it get the lock
>>>>>>>>> during the runtime?
>>>>>>>>>
>>>>>>>> Hi Wenjia,
>>>>>>>>
>>>>>>>> Sorry for that, It is not smc_cdc_rx_handler_rwwi() but
>>>>>>>> smc_cdc_rx_handler();
>>>>>>>>
>>>>>>>> Following is a more specific description of the issues
>>>>>>>>
>>>>>>>>
>>>>>>>> lock_sock()
>>>>>>>> __smc_release
>>>>>>>>
>>>>>>>> smc_cdc_rx_handler()
>>>>>>>> smc_cdc_msg_recv()
>>>>>>>> bh_lock_sock()
>>>>>>>> smc_cdc_msg_recv_action()
>>>>>>>> sock_set_flag(DONE) sock_set_flag(DEAD)
>>>>>>>> __set_bit __set_bit
>>>>>>>> bh_unlock_sock()
>>>>>>>> release_sock()
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> Note : |bh_lock_sock|and |lock_sock|are not mutually exclusive. 
>>>>>>>> They are
>>>>>>>> actually used for different purposes and contexts.
>>>>>>>>
>>>>>>>>
>>>>>>> ok, that's true that |bh_lock_sock|and |lock_sock|are not really 
>>>>>>> mutually
>>>>>>> exclusive. However, since bh_lock_sock() is used, this scenario 
>>>>>>> you described
>>>>>>> above should not happen, because that gets the sk_lock.slock. 
>>>>>>> Following this
>>>>>>> scenarios, IMO, only the following situation can happen.
>>>>>>>
>>>>>>> lock_sock()
>>>>>>> __smc_release
>>>>>>>
>>>>>>> smc_cdc_rx_handler()
>>>>>>> smc_cdc_msg_recv()
>>>>>>> bh_lock_sock()
>>>>>>> smc_cdc_msg_recv_action()
>>>>>>> sock_set_flag(DONE)
>>>>>>> bh_unlock_sock()
>>>>>>> sock_set_flag(DEAD)
>>>>>>> release_sock()
>>>>>> Hi wenjia,
>>>>>>
>>>>>> I think I know what D. Wythe means now, and I think he is right on 
>>>>>> this.
>>>>>>
>>>>>> IIUC, in process context, lock_sock() won't respect bh_lock_sock() 
>>>>>> if it
>>>>>> acquires the lock before bh_lock_sock(). This is how the sock lock 
>>>>>> works.
>>>>>>
>>>>>>       PROCESS CONTEXT INTERRUPT CONTEXT
>>>>>> ------------------------------------------------------------------------
>>>>>> lock_sock()
>>>>>>       spin_lock_bh(&sk->sk_lock.slock);
>>>>>>       ...
>>>>>>       sk->sk_lock.owned = 1;
>>>>>>       // here the spinlock is released
>>>>>>       spin_unlock_bh(&sk->sk_lock.slock);
>>>>>> __smc_release()
>>>>>> bh_lock_sock(&smc->sk);
>>>>>> smc_cdc_msg_recv_action(smc, cdc);
>>>>>> sock_set_flag(&smc->sk, SOCK_DONE);
>>>>>> bh_unlock_sock(&smc->sk);
>>>>>>
>>>>>>       sock_set_flag(DEAD)  <-- Can be before or after 
>>>>>> sock_set_flag(DONE)
>>>>>> release_sock()
>>>>>>
>>>>>> The bh_lock_sock() only spins on sk->sk_lock.slock, which is 
>>>>>> already released
>>>>>> after lock_sock() return. Therefor, there is actually no lock between
>>>>>> the code after lock_sock() and before release_sock() with 
>>>>>> bh_lock_sock()...bh_unlock_sock().
>>>>>> Thus, sock_set_flag(DEAD) won't respect bh_lock_sock() at all, and 
>>>>>> might be
>>>>>> before or after sock_set_flag(DONE).
>>>>>>
>>>>>>
>>>>>> Actually, in TCP, the interrupt context will check 
>>>>>> sock_owned_by_user().
>>>>>> If it returns true, the softirq just defer the process to backlog, 
>>>>>> and process
>>>>>> that in release_sock(). Which avoid the race between softirq and 
>>>>>> process
>>>>>> when visiting the 'struct sock'.
>>>>>>
>>>>>> tcp_v4_rcv()
>>>>>>            bh_lock_sock_nested(sk);
>>>>>>            tcp_segs_in(tcp_sk(sk), skb);
>>>>>>            ret = 0;
>>>>>>            if (!sock_owned_by_user(sk)) {
>>>>>>                    ret = tcp_v4_do_rcv(sk, skb);
>>>>>>            } else {
>>>>>>                    if (tcp_add_backlog(sk, skb, &drop_reason))
>>>>>>                            goto discard_and_relse;
>>>>>>            }
>>>>>>            bh_unlock_sock(sk);
>>>>>>
>>>>>>
>>>>>> But in SMC we don't have a backlog, that means fields in 'struct 
>>>>>> sock'
>>>>>> might all have race, and this sock_set_flag() is just one of the 
>>>>>> cases.
>>>>>>
>>>>>> Best regards,
>>>>>> Dust
>>>>>>
>>>>> I agree on your description above.
>>>>> Sure, the following case 1) can also happen
>>>>>
>>>>> case 1)
>>>>> -------
>>>>> lock_sock()
>>>>> __smc_release
>>>>>
>>>>> sock_set_flag(DEAD)
>>>>> bh_lock_sock()
>>>>> smc_cdc_msg_recv_action()
>>>>> sock_set_flag(DONE)
>>>>> bh_unlock_sock()
>>>>> release_sock()
>>>>>
>>>>> case 2)
>>>>> -------
>>>>> lock_sock()
>>>>> __smc_release
>>>>>
>>>>> bh_lock_sock()
>>>>> smc_cdc_msg_recv_action()
>>>>> sock_set_flag(DONE) sock_set_flag(DEAD)
>>>>> __set_bit __set_bit
>>>>> bh_unlock_sock()
>>>>> release_sock()
>>>>>
>>>>> My point here is that case2) can never happen. i.e that 
>>>>> sock_set_flag(DONE)
>>>>> and sock_set_flag(DEAD) can not happen concurrently. Thus, how could
>>>>> the atomic set help make sure that the Dead flag would not be 
>>>>> overwritten
>>>>> with DONE?
>>>> I agree with you on this. I also don't see using atomic can
>>>> solve the problem of overwriting the DEAD flag with DONE.
>>>>
>>>> I think we need some mechanisms to ensure that sk_flags and other
>>>> struct sock related fields are not modified simultaneously.
>>>>
>>>> Best regards,
>>>> Dust
>>>
>>> It seems that everyone has agrees on that case 2 is impossible. I'm a 
>>> bit confused, why that
>>> sock_set_flag(DONE) and sock_set_flag(DEAD) can not happen 
>>> concurrently. What mechanism
>>> prevents their parallel execution?
>>>
>>> Best wishes,
>>> D. Wythe
>>>
>>>>
>>>
>> In the smc_cdc_rx_handler(), if bh_lock_sock() is got, how could the 
>> sock_set_flag(DEAD) in the __smc_release() modify the flag 
>> concurrently? As I said, that could be just kind of lapse of my 
>> thought, but I still want to make it clarify.
> 
> #define bh_lock_sock(__sk) spin_lock(&((__sk)->sk_lock.slock))
> 
> static inline void lock_sock(struct sock *sk)
> {
>      lock_sock_nested(sk, 0);
> }
> 
> void lock_sock_nested(struct sock *sk, int subclass)
> {
>      might_sleep();
> spin_lock_bh(&sk->sk_lock.slock);
>      if (sk->sk_lock.owned)
>          __lock_sock(sk);
>      sk->sk_lock.owned = 1;
> 
> */spin_unlock(&sk->sk_lock.slock);/*
>      /*
>       * The sk_lock has mutex_lock() semantics here:
>       */
>      mutex_acquire(&sk->sk_lock.dep_map, subclass, 0, _RET_IP_);
>      local_bh_enable();
> }
> 
> 
> It seems that you believe bh_lock_sock() will block the execution of 
> __smc_release(), indicating that you think the spin on slock will block 
> the execution of __smc_release().
> So, you assume that __smc_release() must also spin on slock, right?
> 
That is right what I mean.

> However, lock_sock() releases the slock before returning. You can see it 
> in code above. In other words, __smc_release() will not spin on slock.
> This means that bh_lock_sock() will not block the execution of 
> __smc_release().
> 
Do you mean that the spin_unlock you marked in the code above is to 
release the socket spin lock from __smc_release()?

> Hoping this will helps
> 
> Best wishes,
> D. Wythe
> 
> 
> 
> 
> 

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