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

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Date:   Tue, 7 May 2019 11:54:00 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Eric Dumazet <eric.dumazet@...il.com>, netdev@...r.kernel.org
Cc:     mst@...hat.com, YueHaibing <yuehaibing@...wei.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        "weiyongjun (A)" <weiyongjun1@...wei.com>
Subject: Re: [PATCH net] tuntap: synchronize through tfiles array instead of
 tun->numqueues


On 2019/5/7 上午11:41, Eric Dumazet wrote:
>
> On 5/6/19 11:23 PM, Jason Wang wrote:
>> When a queue(tfile) is detached through __tun_detach(), we move the
>> last enabled tfile to the position where detached one sit but don't
>> NULL out last position. We expect to synchronize the datapath through
>> tun->numqueues. Unfortunately, this won't work since we're lacking
>> sufficient mechanism to order or synchronize the access to
>> tun->numqueues.
>>
>> To fix this, NULL out the last position during detaching and check
>> RCU protected tfile against NULL instead of checking tun->numqueues in
>> datapath.
>>
>> Cc: YueHaibing <yuehaibing@...wei.com>
>> Cc: Cong Wang <xiyou.wangcong@...il.com>
>> Cc: weiyongjun (A) <weiyongjun1@...wei.com>
>> Fixes: c8d68e6be1c3b ("tuntap: multiqueue support")
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> ---
>>   drivers/net/tun.c | 12 +++++++-----
>>   1 file changed, 7 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index e9ca1c0..a64c928 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);
>>   
>>   		--tun->numqueues;
>>   		if (clean) {
>> @@ -1082,7 +1084,7 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
>>   	tfile = rcu_dereference(tun->tfiles[txq]);
>>   
>>   	/* Drop packet if interface is not attached */
>> -	if (txq >= tun->numqueues)
>> +	if (!tfile)
>>   		goto drop;
>>   
>>   	if (!rcu_dereference(tun->steering_prog))
>> @@ -1306,13 +1308,13 @@ static int tun_xdp_xmit(struct net_device *dev, int n,
>>   	rcu_read_lock();
>>   
>>   	numqueues = READ_ONCE(tun->numqueues);
>> -	if (!numqueues) {
>> -		rcu_read_unlock();
>> -		return -ENXIO; /* Caller will free/return all frames */
>> -	}
>>   
> If you remove the test on (!numqueues),
> the following might crash with a divide by zero...


Indeed, let me post V2.

Thanks


>
>>   	tfile = rcu_dereference(tun->tfiles[smp_processor_id() %
>>   					    numqueues]);