From: Michael Brown <mcb30@ipxe.org>
To: paul@xen.org, Wei Liu <wei.liu@kernel.org>,
xen-devel@lists.xenproject.org, netdev@vger.kernel.org,
Paul Durrant <pdurrant@amazon.com>
Subject: Re: xen-netback hotplug-status regression bug
Date: Tue, 13 Apr 2021 11:48:21 +0100 [thread overview]
Message-ID: <58ccc3b7-9ccb-b9bf-84e7-4a023ccb5c56@ipxe.org> (raw)
In-Reply-To: <f469cdee-f97e-da3f-bcab-0be9ed8cd836@xen.org>
On 13/04/2021 08:12, Paul Durrant wrote:
>> If the frontend subsequently disconnects and reconnects (e.g.
>> transitions through Closed->Initialising->Connected) then:
>>
>> - Nothing recreates "hotplug-status"
>>
>> - When the frontend re-enters Connected state, connect() sets up a
>> watch on "hotplug-status" again
>>
>> - The callback hotplug_status_changed() is never triggered, and so the
>> backend device never transitions to Connected state.
>
> That's not how I read it. Given that "hotplug-status" is removed by the
> call to hotplug_status_changed() then the next call to connect() should
> fail to register the watch and 'have_hotplug_status_watch' should be 0.
> Thus backend_switch_state() should not defer the transition to
> XenbusStateConnected in any subsequent interaction with the frontend.
Thank you for the reply. I've tested and confirmed my initial
hypothesis: the call to xenbus_watch_pathfmt() succeeds even if the node
does not exist.
I confirmed this with ftrace using:
cd /sys/kernel/debug/tracing
echo function_graph > current_tracer
echo set_backend_state > set_ftrace_filter
echo xenbus_watch_pathfmt >> set_ftrace_filter
echo register_xenbus_watch >> set_ftrace_filter
echo xenbus_dev_fatal >> set_ftrace_filter
On the second time that the frontend transitions to Connected, this
produced the trace:
set_backend_state [xen_netback]() {
register_xenbus_watch();
register_xenbus_watch();
xenbus_watch_pathfmt() {
register_xenbus_watch();
}
}
which seems to confirm that the error path in xenbus_watch_path() is
*not* taken, i.e. that the call to register_xenbus_watch() succeeded
even though the node did not exist.
Other observations also seem to confirm this behaviour:
- Running "xenstore ls" in dom0 confirms that on the second frontend
transition to Connected, the frontend state is indeed Connected (4) but
the backend state remains in InitWait (2)
- Running "xenstore watch
/local/domain/0/backend/vif/<domU>/0/hotplug-status" *before* starting
the domU confirms that it is possible to create a watch on a node that
does not (yet) exist, and that the watch *is* notified when the node is
later created.
> Are you seeing the watch successfully re-registered even though the node
> does not exist? Perhaps there has been a change in xenstore behaviour?
So, the TL;DR is that yes, the watch does successfully register even
though the node does not exist.
From a quick look through the xenstored source, it looks as though the
only check on the node name is the call to is_valid_nodename(), which
seems to perform a syntactic validity check only. I can't immediately
find any commit that would have changed this behaviour.
Thanks,
Michael
next prev parent reply other threads:[~2021-04-13 10:48 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 [this message]
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
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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