netdev - Re: [PATCH v2 net] xen: netback: read hotplug script once at start of day.

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Date:	Mon, 1 Jun 2015 12:24:22 +0100
From:	Wei Liu <wei.liu2@...rix.com>
To:	Ian Campbell <ian.campbell@...rix.com>
CC:	<xen-devel@...ts.xen.org>, <netdev@...r.kernel.org>,
	<wei.liu2@...rix.com>, <david.vrabel@...rix.com>
Subject: Re: [PATCH v2 net] xen: netback: read hotplug script once at start
 of day.

On Mon, Jun 01, 2015 at 11:30:24AM +0100, Ian Campbell wrote:
> When we come to tear things down in netback_remove() and generate the
> uevent it is possible that the xenstore directory has already been
> removed (details below).
> 
> In such cases netback_uevent() won't be able to read the hotplug
> script and will write a xenstore error node.
> 
> A recent change to the hypervisor exposed this race such that we now
> sometimes lose it (where apparently we didn't ever before).
> 
> Instead read the hotplug script configuration during setup and use it
> for the lifetime of the backend device.
> 
> The apparently more obvious fix of moving the transition to
> state=Closed in netback_remove() to after the uevent does not work
> because it is possible that we are already in state=Closed (in
> reaction to the guest having disconnected as it shutdown). Being
> already in Closed means the toolstack is at liberty to start tearing
> down the xenstore directories. In principal it might be possible to
> arrange to unregister the device sooner (e.g on transition to Closing)
> such that xenstore would still be there but this state machine is
> fragile and prone to anger...
> 
> A modern Xen system only relies on the hotplug uevent for driver
> domains, when the backend is in the same domain as the toolstack it
> will run the necessary setup/teardown directly in the correct sequence
> wrt xenstore changes.
> 
> Signed-off-by: Ian Campbell <ian.campbell@...rix.com>

Acked-by: Wei Liu <wei.liu2@...rix.com>
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