netdev - Re: [PATCH net-next] virtio-net: invoke zerocopy callback on xmit path if no tx napi

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Date:   Mon, 21 Aug 2017 21:58:45 +0900
From:   Koichiro Den <den@...ipeden.com>
To:     Jason Wang <jasowang@...hat.com>, mst@...hat.com
Cc:     virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] virtio-net: invoke zerocopy callback on xmit
 path if no tx napi

On Mon, 2017-08-21 at 20:33 +0800, Jason Wang wrote:
> 
> On 2017年08月19日 14:38, Koichiro Den wrote:
> > Facing the possible unbounded delay relying on freeing on xmit path,
> > we also better to invoke and clear the upper layer zerocopy callback
> > beforehand to keep them from waiting for unbounded duration in vain.
> > For instance, this removes the possible deadlock in the case that the
> > upper layer is a zerocopy-enabled vhost-net.
> > This does not apply if napi_tx is enabled since it will be called in
> > reasonale time.
> > 
> > Signed-off-by: Koichiro Den <den@...ipeden.com>
> > ---
> >   drivers/net/virtio_net.c | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> > 
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 4302f313d9a7..f7deaa5b7b50 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -1290,6 +1290,14 @@ static netdev_tx_t start_xmit(struct sk_buff *skb,
> > struct net_device *dev)
> >   
> >   	/* Don't wait up for transmitted skbs to be freed. */
> >   	if (!use_napi) {
> > +		if (skb_shinfo(skb)->tx_flags & SKBTX_DEV_ZEROCOPY) {
> > +			struct ubuf_info *uarg;
> > +			uarg = skb_shinfo(skb)->destructor_arg;
> > +			if (uarg->callback)
> > +			    uarg->callback(uarg, true);
> > +			skb_shinfo(skb)->destructor_arg = NULL;
> > +			skb_shinfo(skb)->tx_flags &= ~SKBTX_DEV_ZEROCOPY;
> > +		}
> >   		skb_orphan(skb);
> >   		nf_reset(skb);
> >   	}
> 
> 
> Interesting, deadlock could be treated as a a radical case of the 
> discussion here https://patchwork.kernel.org/patch/3787671/.
Thanks for letting me know this one. I am going to read it.
> 
> git grep tells more similar skb_orphan() cases. Do we need to change 
> them all (or part)?
Maybe, even though it seems less likely that we may meet it than the one I
described as an example, such as virtio-net sandwiched between vhost-net.
> 
> Actually, we may meet similar issues at many other places (e.g netem). 
> Need to consider a complete solution for this. Figuring out all places 
> that could delay a packet is a method.
Okay I will do it and post the result and a suggestion if possible. Thanks.
> 
> Thanks