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

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Date:   Fri, 18 May 2018 22:00:31 +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 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