netdev - Re: [PATCH net V2] tuntap: synchronize through tfiles array instead of tun->numqueues

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Message-ID: <a1ef0c0d-d67c-8888-91e6-2819e8c45489@redhat.com>
Date:   Tue, 7 May 2019 14:19:20 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        YueHaibing <yuehaibing@...wei.com>,
        "weiyongjun (A)" <weiyongjun1@...wei.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net V2] tuntap: synchronize through tfiles array instead
 of tun->numqueues


On 2019/5/7 下午12:54, Cong Wang wrote:
> On Mon, May 6, 2019 at 9:03 PM Jason Wang <jasowang@...hat.com> wrote:
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index e9ca1c0..32a0b23 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -700,6 +700,8 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
>>                                     tun->tfiles[tun->numqueues - 1]);
>>                  ntfile = rtnl_dereference(tun->tfiles[index]);
>>                  ntfile->queue_index = index;
>> +               rcu_assign_pointer(tun->tfiles[tun->numqueues - 1],
>> +                                  NULL);
>>
> How does this work? Existing readers could still read this
> tun->tfiles[tun->numqueues - 1] before you NULL it. And,
> _if_ the following sock_put() is the one frees it, you still miss
> a RCU grace period.
>
>                  if (clean) {
>                          RCU_INIT_POINTER(tfile->tun, NULL);
>                          sock_put(&tfile->sk);
>
>
> Thanks.


My understanding is the socket will never be freed for this sock_put(). 
We just drop an extra reference count we held when the socket was 
attached to the netdevice (there's a sock_hold() in tun_attach()). The 
real free should happen at another sock_put() in the end of this function.

Thanks