Re: [Xen-devel] xl shutdown --wait "racy"

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2014-04-16 at 16:55 +0200, Sander Eikelenboom wrote:
> Wednesday, April 16, 2014, 4:33:30 PM, you wrote:
> 
> > On Wed, 2014-04-16 at 16:26 +0200, Sander Eikelenboom wrote:
> >> Wednesday, April 16, 2014, 4:13:59 PM, you wrote:
> >> 
> >> > On Wed, 2014-04-16 at 16:08 +0200, Sander Eikelenboom wrote:
> >> >> Hi Ian (C|J) Konrad,
> >> >> 
> >> >> I'm currently trying to workaround the pci-(detach|assignable-remove) 
> >> >> issues i 
> >> >> reported earlier. 
> >> >> 
> >> >> The workaround i thought of was:
> >> >> - shutting down the guest
> >> >> - starting it without 1 of the original devices passed through
> >> >> - use xl pci-assignable-remove and bind the device to the dom0 driver.
> >> >> 
> >> >> But during this i noticed that a "xl shutdown --wait" does wait .. but 
> >> >> returns:
> >> >> - Before the domain is removed from for instance "xl list", it still 
> >> >> listed there in 
> >> >> "--ps--" state.
> >> >> - before pciback has done it's restore config space magic.
> >> >> 
> >> >> So it seems the wait loop is exiting somewhat prematurely, is this 
> >> >> expected ? 
> >> 
> >> > It is waiting for the domain to be shutdown (state 's') not for the
> >> > domain to be destroyed. So it's doing what it said it would (I
> >> > appreciate you might not find this distinction helpful under the
> >> > circumstances...)
> >> 
> >> It's at least not entirely what i expected ;-)
> >> 
> >> Is it because there can be different "follow-up actions" due to the 
> >> "on_poweroff=" config option ?
> 
> > Not really, those are somewhat unrelated.
> 
> > shutdown and destroy are two distinct events. Once a domain has shutdown
> > (called the shutdown hypercall etc) it goes into state "shutdown" and an
> > event is generated from the hypervisor to the toolstack. The toolstack's
> > response to this is to actually destroy the domain, that is to tear down
> > the resources it is using etc.
> 
> > on_* only matter for the destroy phase since they tell the toolstack
> > what it should do (restart, preserve, really destroy etc).
> 
> Hmm ok, it should be called "--wait_until_halfway" then ;-)

;-)

> On the more serious side .. would patches be accepted that:
> 
> a) differentiate when it returns from waiting based on the on_*
> 
>         preserve: this could probably stay as is .. after the shutdown event
>         destroy:
>         restart:
>         rename-restart:
>         coredump-destroy:
>         coredump-restart:
> 
>         for the other ones .. i don't know if there actually are events in 
> libxl 
>         that could be 'easily' coupled ?

Might be tricky, since on_* is processed by the daemonised xl which is
monitoring the domain, not the xl shutdown process.

> b) make it possible for the xl commandline to overrule the on_* from the 
> configfile

I guess you mean the xl shutdown command. This will also be tricky, for
the same reasons as a.

> c) also introduce a -w/--wait for xl destroy

Yes.

I'll add:

d) Make "xl shutdown --wait" actually wait for the domain to be
destroyed.

Probably, assuming that is possible (I'm concerned about races in the
implementation of this...). Might also interact weirdly with on_* I
suppose.

Ian.


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