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

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Date:   Fri, 18 May 2018 22:11:54 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Hannes Frederic Sowa <hannes@...essinduktion.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net] tuntap: raise EPOLLOUT on device up



On 2018年05月18日 22:06, Michael S. Tsirkin wrote:
> On Fri, May 18, 2018 at 10:00:31PM +0800, Jason Wang wrote:
>>
>> On 2018年05月18日 21:26, Jason Wang wrote:
>>>
>>> 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@...essinduktion.org>
>>>>> Cc: Eric Dumazet <edumazet@...gle.com>
>>>>> Fixes: 1bd4978a88ac2 ("tun: honor IFF_UP in tun_get_user()")
>>>>> Signed-off-by: Jason Wang <jasowang@...hat.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
>> Looks no need, vhost will poll socket after it see EIO. So we are ok here?
>>
>> Thanks
> In fact I don't even understand why does this help any longer.
>

We disable tx polling and only enable it on demand for a better rx 
performance. You may want to have a look at :

commit feb8892cb441c742d4220cf7ced001e7fa070731
Author: Jason Wang <jasowang@...hat.com>
Date:   Mon Nov 13 11:45:34 2017 +0800

     vhost_net: conditionally enable tx polling

Thanks