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

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Date:   Fri, 18 May 2018 17:46:17 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...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 Fri, May 18, 2018 at 10:11:54PM +0800, Jason Wang wrote:
> 
> 
> 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


Question is, what looks at SOCKWQ_ASYNC_NOSPACE.
I think it's tested when packet is transmitted,
but there is no guarantee here any packet will
ever be transmitted.

-- 
MST