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Re: [Xen-devel] Xen scripts rewrite

On 01.03.2012 18:32, Mark wrote:
>>> I am trying to wrap my head around the xen scripts
>>> in /etc/xen/scripts. I am currently looking at locking.sh but I
>>> don't really understand what it is doing.
>>> I see that claim_lock has a while loop, it seems to wait until it
>>> can write its own PID to $lockdir/owner. If the file $lockdir/owner
>>> exists, check if the pid is still running. If it's still running
>>> when the loop is done, steal the lock by writing $$ to
>>> $lockdir/owner anyway.
>>> Anyway. Can someone please further explain this file in plain
>>> English and detail. Am I right with my above assumptions, and when
>>> are locks even used?
>>> As said, I request as much detail as possible - the rewrite is to
>>> improve efficiency, readability and user customisation. As it is now
>>> you have too much functions that confuse me silly.
>>> If I am succesful I will share the outcome, so there's an incentive
>>> if you need one.
>> Current locking mechanism have some problems. I've written about it
>> here:
>> http://lists.xen.org/archives/html/xen-devel/2011-07/msg00182.html
> Alright, thanks.
> I really do need to understand the purpose of locking and when Xen
> does what with regards to locking. 

These scripts are called by udev (from /etc/udev/rules.d/xen-backend.rules) as
result of appearing new backend device. Toolstack (eg. xend) uses it to finish
setup backend (eg. attach vif interface to brigde, or prepare /dev/loopX for
block backend). They are called simultaneously, so some locking is required
(eg. to ensure that no two script choose same loop device number).
With xl it is similar, at least for network backend. AFAIK xl doesn't support
hotplug script for block backend yet (all setup is done by libxl internally).

> So, my request for a detailed
> explanation still stands. And I have yet to understand whether xl uses
> these scripts, because I am still using xm for the moment.
> I have a bit of success already because with a little bit of hacking
> here and there I have managed to write a script for use with
> openvswitch. But I don't want to release my patches in small steps, a
> lot has to be done first.
> So who's gonna shed some light on this? And is anyone interested to
> join the fun? :)
> Mark

Best Regards / Pozdrawiam,
Marek Marczykowski
Invisible Things Lab

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