netdev - Re: [PATCH net 5/5] net/smc: put sk reference if close work was canceled

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Date: Thu, 19 Oct 2023 15:33:04 +0800
From: "D. Wythe" <alibuda@...ux.alibaba.com>
To: Wenjia Zhang <wenjia@...ux.ibm.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 5/5] net/smc: put sk reference if close work was
 canceled



On 10/19/23 4:26 AM, Wenjia Zhang wrote:
>
>
> On 17.10.23 04:06, D. Wythe wrote:
>>
>>
>> On 10/13/23 3:04 AM, Wenjia Zhang wrote:
>>>
>>>
>>> On 11.10.23 09:33, D. Wythe wrote:
>>>> From: "D. Wythe" <alibuda@...ux.alibaba.com>
>>>>
>>>> Note that we always hold a reference to sock when attempting
>>>> to submit close_work. 
>>> yes
>>> Therefore, if we have successfully
>>>> canceled close_work from pending, we MUST release that reference
>>>> to avoid potential leaks.
>>>>
>>> Isn't the corresponding reference already released inside the 
>>> smc_close_passive_work()?
>>>
>>
>> Hi Wenjia,
>>
>> If we successfully cancel the close work from the pending state,
>> it means that smc_close_passive_work() has never been executed.
>>
>> You can find more details here.
>>
>> /**
>> * cancel_work_sync - cancel a work and wait for it to finish
>> * @work:the work to cancel
>> *
>> * Cancel @work and wait for its execution to finish. This function
>> * can be used even if the work re-queues itself or migrates to
>> * another workqueue. On return from this function, @work is
>> * guaranteed to be not pending or executing on any CPU.
>> *
>> * cancel_work_sync(&delayed_work->work) must not be used for
>> * delayed_work's. Use cancel_delayed_work_sync() instead.
>> *
>> * The caller must ensure that the workqueue on which @work was last
>> * queued can't be destroyed before this function returns.
>> *
>> * Return:
>> * %true if @work was pending, %false otherwise.
>> */
>> boolcancel_work_sync(structwork_struct *work)
>> {
>> return__cancel_work_timer(work, false);
>> }
>>
>> Best wishes,
>> D. Wythe
> As I understand, queue_work() would wake up the work if the work is 
> not already on the queue. And the sock_hold() is just prio to the 
> queue_work(). That means, cancel_work_sync() would cancel the work 
> either before its execution or after. If your fix refers to the former 
> case, at this moment, I don't think the reference can be hold, thus it 
> is unnecessary to put it.
>>

I am quite confuse about why you think when we cancel the work before 
its execution,
the reference can not be hold ?


Perhaps the following diagram can describe the problem in better way :

smc_close_cancel_work
smc_cdc_msg_recv_action


sock_hold
queue_work
                                                                if 
(cancel_work_sync())        // successfully cancel before execution
sock_put()                        //  need to put it since we already 
hold a ref before   queue_work()


>>>> Fixes: 42bfba9eaa33 ("net/smc: immediate termination for SMCD link 
>>>> groups")
>>>> Signed-off-by: D. Wythe <alibuda@...ux.alibaba.com>
>>>> ---
>>>>   net/smc/smc_close.c | 3 ++-
>>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/net/smc/smc_close.c b/net/smc/smc_close.c
>>>> index 449ef45..10219f5 100644
>>>> --- a/net/smc/smc_close.c
>>>> +++ b/net/smc/smc_close.c
>>>> @@ -116,7 +116,8 @@ static void smc_close_cancel_work(struct 
>>>> smc_sock *smc)
>>>>       struct sock *sk = &smc->sk;
>>>>         release_sock(sk);
>>>> -    cancel_work_sync(&smc->conn.close_work);
>>>> +    if (cancel_work_sync(&smc->conn.close_work))
>>>> +        sock_put(sk);
>>>>       cancel_delayed_work_sync(&smc->conn.tx_work);
>>>>       lock_sock(sk);
>>>>   }
>>