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Date:   Wed, 8 May 2019 12:30:00 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     netdev@...r.kernel.org, YueHaibing <yuehaibing@...wei.com>,
        Cong Wang <xiyou.wangcong@...il.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 2019/5/8 下午12:16, Michael S. Tsirkin wrote:
> On Tue, May 07, 2019 at 12:03:36AM -0400, 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>
>> Cc: Eric Dumazet <eric.dumazet@...il.com>
>> Fixes: c8d68e6be1c3b ("tuntap: multiqueue support")
>> Signed-off-by: Jason Wang <jasowang@...hat.com>
>> ---
>> Changes from V1:
>> - keep the check in tun_xdp_xmit()
>> ---
>>   drivers/net/tun.c | 8 +++++++-
>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> 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);
>>   
>>   		--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))
> Hmm don't we need to range check txq?


Looks not since tun_select_queue will always return a value which is 
less than MAX_TAP_QUEUES. And we NULL out the last enabled queue in 
tun_detach().

Thanks


>
>
>> @@ -1313,6 +1315,10 @@ static int tun_xdp_xmit(struct net_device *dev, int n,
>>   
>>   	tfile = rcu_dereference(tun->tfiles[smp_processor_id() %
>>   					    numqueues]);
>> +	if (!tfile) {
>> +		rcu_read_unlock();
>> +		return -ENXIO; /* Caller will free/return all frames */
>> +	}
>>   
>>   	spin_lock(&tfile->tx_ring.producer_lock);
>>   	for (i = 0; i < n; i++) {
>> -- 
>> 1.8.3.1

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