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Message-ID: <76715d1b-00a9-8da2-2af5-7a2198de9b8a@gmail.com>
Date:   Fri, 22 Feb 2019 17:16:37 -0500
From:   David Ahern <dsahern@...il.com>
To:     wenxu <wenxu@...oud.cn>, davem@...emloft.net,
        netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2] ip_tunnel: Add dst_cache management
 lwtunnel_state of ip tunnel

On 2/22/19 10:21 AM, wenxu wrote:
> On 2019/2/22 下午11:03, David Ahern wrote:
>> On 2/21/19 11:14 PM, wenxu wrote:
>>> build_state in the rcu_read_lock and disable the preempt
>>>
>>>  rcu_read_lock();
>>>     ops = rcu_dereference(lwtun_encaps[encap_type]);
>>>     if (likely(ops && ops->build_state && try_module_get(ops->owner))) {
>>>         found = true;
>>>         ret = ops->build_state(encap, family, cfg, lws, extack);
>>>         if (ret)
>>>             module_put(ops->owner);
>>>     }  
>>>     rcu_read_unlock();
>>>
>> Missed that.
>>
>> Once a reference is taken the rcu_read_lock can be dropped before
>> calling build_state allowing the allocations to be GFP_KERNEL.
>>
> I don't think the rcu_read_lock can be dropped. The whole operation of the reference should be protect
> 
> in the rcu_read_lock that ensure the ops will not be destroy (if it will be)
> 

If 'try_module_get(ops->owner)' succeeds then a reference is held so the
ops will not go away.

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