Re: [Xen-devel] Xenbus watch firing over and over

[Ewan Mellor <ewan@xxxxxxxxxxxxx>]

On Wed, Jan 03, 2007 at 07:16:03PM +0100, Jacob Gorm Hansen wrote:

> hi,
> I am trying to add xenbus watch support to mini-os, but am getting into
> a endless loop of watches firing over and over. Below is an excerpt from
> the xenstore tracefile:
> 
> IN  0x805acc8 20070103 19:32:18 WATCH
> (/local/domain/0/backend/vif/26/0/state 00043818 )
> CREATE watch 0x805a8b8
> OUT 0x805acc8 20070103 19:32:18 WATCH (OK )
> OUT 0x805acc8 20070103 19:32:18 WATCH_EVENT
> (/local/domain/0/backend/vif/26/0/state 00043818 )
> IN  0x805acc8 20070103 19:32:18 UNWATCH
> (/local/domain/0/backend/vif/26/0/state 00043818 )
> DESTROY watch 0x805a8b8
> OUT 0x805acc8 20070103 19:32:18 UNWATCH (OK )
> IN  0x805acc8 20070103 19:32:18 READ
> (/local/domain/0/backend/vif/26/0/state )
> OUT 0x805acc8 20070103 19:32:18 READ (5)
> IN  0x805acc8 20070103 19:32:18 WATCH
> (/local/domain/0/backend/vif/26/0/state 00043818 )
> CREATE watch 0x806af60
> OUT 0x805acc8 20070103 19:32:18 WATCH (OK )
> OUT 0x805acc8 20070103 19:32:18 WATCH_EVENT
> (/local/domain/0/backend/vif/26/0/state 00043818 )
> IN  0x805acc8 20070103 19:32:18 UNWATCH
> (/local/domain/0/backend/vif/26/0/state 00043818 )
> DESTROY watch 0x806af60
> OUT 0x805acc8 20070103 19:32:18 UNWATCH (OK )
> IN  0x805acc8 20070103 19:32:18 READ
> (/local/domain/0/backend/vif/26/0/state )
> OUT 0x805acc8 20070103 19:32:18 READ (5)
> IN  0x805acc8 20070103 19:32:18 WATCH
> (/local/domain/0/backend/vif/26/0/state 00043818 )
> 
> ...
> 
> The same watch seems to be firing again and again, even though noone
> appears to be writing it. Should I some ack the watch, or is this a bug
> in xenstored?

As far as xenstored is concerned, someone is explicitly deleting and
reregistering that watch -- I don't think this is a bug in xenstored.  You
should expect to receive a watch event as soon as a watch is registered -- it
looks to me like someone is receiving that event, then unwatching the node,
reading its value, and reregistering the watch.

Ewan.

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