netdev - Re: [PATCH net-next v2] ip_tunnel: Add dst_cache management lwtunnel

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Date:   Fri, 22 Feb 2019 23:21:14 +0800
From:   wenxu <wenxu@...oud.cn>
To:     David Ahern <dsahern@...il.com>, 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 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)