netdev - Re: [PATCH net] failover: eliminate callback hell

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Date:   Tue, 5 Jun 2018 23:00:38 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     "Samudrala, Sridhar" <sridhar.samudrala@...el.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>, kys@...rosoft.com,
        haiyangz@...rosoft.com, davem@...emloft.net,
        netdev@...r.kernel.org, Stephen Hemminger <sthemmin@...rosoft.com>
Subject: Re: [PATCH net] failover: eliminate callback hell

On Tue, 5 Jun 2018 22:39:12 -0700
"Samudrala, Sridhar" <sridhar.samudrala@...el.com> wrote:

> On 6/5/2018 8:51 PM, Stephen Hemminger wrote:
> > On Tue, 5 Jun 2018 16:52:22 -0700
> > "Samudrala, Sridhar" <sridhar.samudrala@...el.com> wrote:
> >  
> >> On 6/5/2018 2:52 PM, Stephen Hemminger wrote:  
> >>> On Tue, 5 Jun 2018 22:38:43 +0300
> >>> "Michael S. Tsirkin" <mst@...hat.com> wrote:
> >>>     
> >>>>> See:
> >>>>>      https://patchwork.ozlabs.org/patch/851711/  
> >>>> Let me try to summarize that:
> >>>>
> >>>> 	You wanted to speed up the delayed link up.  You had an idea to
> >>>> 	additionally take link up when userspace renames the interface (standby
> >>>> 	one which is also the failover for netvsc).
> >>>>
> >>>> 	But userspace might not do any renames, in which case there will
> >>>> 	still be the delay, and so this never got applied.
> >>>>
> >>>> 	Is this a good summary?
> >>>>
> >>>> Davem said delay should go away completely as it's not robust, and I
> >>>> think I agree.  So I don't think we should make all failover users use
> >>>> delay. IIUC failover kept a delay option especially for netvsc to
> >>>> minimize the surprise factor. Hopefully we can come up with
> >>>> something more robust and drop that option completely.  
> >>> The timeout was the original solution to how to complete setup after
> >>> userspace has had a chance to rename the device. Unfortunately, the whole network
> >>> device initialization (cooperation with udev and userspace) is a a mess because
> >>> there is no well defined specification, and there are multiple ways userspace
> >>> does this in old and new distributions.  The timeout has its own issues
> >>> (how long, handling errors during that window, what if userspace modifies other
> >>> device state); and open to finding a better solution.
> >>>
> >>> My point was that if name change can not be relied on (or used) by netvsc,
> >>> then we can't allow it for net_failover either.  
> >> I think the push back was with the usage of the delay, not bringing up the primary/standby
> >> device in the name change event handler.
> >> Can't netvsc use this mechanism instead of depending on the delay?
> >>
> >>  
> > The patch that was rejected for netvsc was about using name change.
> > Also, you can't depend on name change; you still need a timer. Not all distributions
> > change name of devices. Or user has blocked that by udev rules.  
> 
> In the net_failover_slave_register() we do a dev_open() and ignore any failure due to
> EBUSY and do another dev_open() in the name change event handler.
> If the name is not expected to change, i would think the dev_open() at the time of
> register will succeed.

The problem is your first dev_open will bring device up and lockout
udev from changing the network device name.