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From: Paul Durrant <xadimgnik@gmail.com>
To: Michael Brown <mcb30@ipxe.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 08:12:44 +0100	[thread overview]
Message-ID: <f469cdee-f97e-da3f-bcab-0be9ed8cd836@xen.org> (raw)
In-Reply-To: <afedd7cb-a291-e773-8b0d-4db9b291fa98@ipxe.org>

On 10/04/2021 19:25, Michael Brown wrote:
> Commit https://github.com/torvalds/linux/commit/1f25657 ("xen-netback: 
> remove 'hotplug-status' once it has served its purpose") seems to have 
> introduced a regression that prevents a vif frontend from transitioning 
> more than once into Connected state.
> 
> As far as I can tell:
> 
> - The defined vif script (e.g. /etc/xen/scripts/vif-bridge) executes 
> only once, at domU startup, and sets 
> backend/vif/<domU>/0/hotplug-status="connected"
> 
> - When the frontend first enters Connected state, 
> drivers/net/xen-netback/xenbus.c's connect() sets up a watch on 
> "hotplug-status" with the callback function hotplug_status_changed()
> 
> - When hotplug_status_changed() is triggered by the watch, it 
> transitions the backend to Connected state and calls xenbus_rm() to 
> delete the "hotplug-status" attribute.
> 
> 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.


> 
> Reverting the commit would fix this bug, but would obviously also 
> reintroduce the race condition that the commit was designed to avoid.
> 
> I'm happy to put together a patch, if one of the maintainers could 
> suggest a sensible design approach.
>

Are you seeing the watch successfully re-registered even though the node 
does not exist? Perhaps there has been a change in xenstore behaviour?

   Paul

> I'm not a list member, so please CC me directly on replies.
> 
> Thanks,
> 
> Michael



  reply	other threads:[~2021-04-13  7:13 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-10 18:25 Michael Brown
2021-04-13  7:12 ` Paul Durrant [this message]
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
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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