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

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Message-ID: <20180522010700-mutt-send-email-mst@kernel.org>
Date:   Tue, 22 May 2018 01:08:23 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     David Miller <davem@...emloft.net>
Cc:     jasowang@...hat.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, hannes@...essinduktion.org,
        edumazet@...gle.com
Subject: Re: [PATCH net] tuntap: raise EPOLLOUT on device up

On Mon, May 21, 2018 at 11:47:42AM -0400, David Miller wrote:
> From: Jason Wang <jasowang@...hat.com>
> Date: Fri, 18 May 2018 21:00:43 +0800
> 
> > 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>
> 
> I'm no so sure what to do with this patch.
> 
> Like Michael says, this flag bit is only checks upon transmit which
> may or may not happen after this point.  It doesn't seem to be
> guaranteed.

Jason, can't we detect a link up transition and respond accordingly?
What do you think?

-- 
MST