Re: [PATCH net] tuntap: raise EPOLLOUT on device up

[Jason Wang <jasowang@redhat.com>]

On 2018年05月18日 21:13, Michael S. Tsirkin wrote:
> On Fri, May 18, 2018 at 09:00:43PM +0800, Jason Wang wrote:
>> We return -EIO on device down but can not raise EPOLLOUT after it was
>> up. This may confuse user like vhost which expects tuntap to raise
>> EPOLLOUT to re-enable its TX routine after tuntap is down. This could
>> be easily reproduced by transmitting packets from VM while down and up
>> the tap device. Fixing this by set SOCKWQ_ASYNC_NOSPACE on -EIO.
>>
>> Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
>> Cc: Eric Dumazet <edumazet@google.com>
>> Fixes: 1bd4978a88ac2 ("tun: honor IFF_UP in tun_get_user()")
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>> ---
>>   drivers/net/tun.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index d45ac37..1b29761 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -1734,8 +1734,10 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
>>   	int skb_xdp = 1;
>>   	bool frags = tun_napi_frags_enabled(tun);
>>   
>> -	if (!(tun->dev->flags & IFF_UP))
>> +	if (!(tun->dev->flags & IFF_UP)) {
> Isn't this racy?  What if flag is cleared at this point?

I think you mean "set at this point"? Then yes, so we probably need to 
set the bit during tun_net_close().

Thanks

>> +		set_bit(SOCKWQ_ASYNC_NOSPACE, &tfile->socket.flags);
>>   		return -EIO;
>> +	}
>>   
>>   	if (!(tun->flags & IFF_NO_PI)) {
>>   		if (len < sizeof(pi))
>> -- 
>> 2.7.4