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From: "Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>
To: Michael Brown <mbrown@fensystems.co.uk>
Cc: paul@xen.org, xen-devel@lists.xenproject.org,
	netdev@vger.kernel.org, wei.liu@kernel.org, pdurrant@amazon.com
Subject: Re: [PATCH] xen-netback: Check for hotplug-status existence before watching
Date: Mon, 10 May 2021 20:53:02 +0200	[thread overview]
Message-ID: <YJmBDpqQ12ZBGf58@mail-itl> (raw)
In-Reply-To: <404130e4-210d-2214-47a8-833c0463d997@fensystems.co.uk>

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On Mon, May 10, 2021 at 07:47:01PM +0100, Michael Brown wrote:
> On 10/05/2021 19:32, Marek Marczykowski-Górecki wrote:
> > On Tue, Apr 13, 2021 at 04:25:12PM +0100, Michael Brown wrote:
> > > The logic in connect() is currently written with the assumption that
> > > xenbus_watch_pathfmt() will return an error for a node that does not
> > > exist.  This assumption is incorrect: xenstore does allow a watch to
> > > be registered for a nonexistent node (and will send notifications
> > > should the node be subsequently created).
> > > 
> > > As of commit 1f2565780 ("xen-netback: remove 'hotplug-status' once it
> > > has served its purpose"), this leads to a failure when a domU
> > > transitions into XenbusStateConnected more than once.  On the first
> > > domU transition into Connected state, the "hotplug-status" node will
> > > be deleted by the hotplug_status_changed() callback in dom0.  On the
> > > second or subsequent domU transition into Connected state, the
> > > hotplug_status_changed() callback will therefore never be invoked, and
> > > so the backend will remain stuck in InitWait.
> > > 
> > > This failure prevents scenarios such as reloading the xen-netfront
> > > module within a domU, or booting a domU via iPXE.  There is
> > > unfortunately no way for the domU to work around this dom0 bug.
> > > 
> > > Fix by explicitly checking for existence of the "hotplug-status" node,
> > > thereby creating the behaviour that was previously assumed to exist.
> > 
> > This change is wrong. The 'hotplug-status' node is created _only_ by a
> > hotplug script and done so when it's executed. When kernel waits for
> > hotplug script to be executed it waits for the node to _appear_, not
> > _change_. So, this change basically made the kernel not waiting for the
> > hotplug script at all.
> 
> That doesn't sound plausible to me.  In the setup as you describe, how is
> the kernel expected to differentiate between "hotplug script has not yet
> created the node" and "hotplug script does not exist and will therefore
> never create any node"?

Is the later valid at all? From what I can see in the toolstack, it
always sets some hotplug script (if not specified explicitly - then
"vif-bridge"),

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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  reply	other threads:[~2021-05-10 18:53 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-10 18:25 xen-netback hotplug-status regression bug Michael Brown
2021-04-13  7:12 ` Paul Durrant
2021-04-13 10:48   ` Michael Brown
2021-04-13 10:55     ` Paul Durrant
2021-04-13 15:14       ` Michael Brown
2021-04-13 15:25       ` [PATCH] xen-netback: Check for hotplug-status existence before watching Michael Brown
2021-04-13 19:12         ` Paul Durrant
2021-04-13 22:30         ` patchwork-bot+netdevbpf
2021-05-10 18:32         ` Marek Marczykowski-Górecki
2021-05-10 18:47           ` Michael Brown
2021-05-10 18:53             ` Marek Marczykowski-Górecki [this message]
2021-05-10 19:06               ` Michael Brown
2021-05-10 19:42                 ` Marek Marczykowski-Górecki
2021-05-11  7:06                   ` Durrant, Paul
2021-05-11 10:40                     ` Marek Marczykowski-Górecki
2021-05-11 10:45                       ` Marek Marczykowski-Górecki
2021-05-11 12:46                         ` Durrant, Paul
2021-05-17 21:43                           ` Marek Marczykowski-Górecki
2021-05-17 21:51                             ` Michael Brown
2021-05-17 21:58                               ` Marek Marczykowski-Górecki
2021-05-18  6:57                             ` Paul Durrant
2021-05-18  9:18                               ` Marek Marczykowski-Górecki
     [not found]                                 ` <887f9533f5c54bfabfbff7231eb99b08@EX13D32EUC003.ant.amazon.com>
     [not found]                                   ` <YKOMpXwcnr9QiXy8@mail-itl>
     [not found]                                     ` <2c23e102b6254e42877eb1e8fe68a4f7@EX13D32EUC003.ant.amazon.com>
2021-05-18 10:42                                       ` Marek Marczykowski-Górecki

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