Re: [PATCH] libxl: do not fail device removal if backend domain is gone - Marek Marczykowski-Górecki

From: "Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	xen-devel@lists.xen.org
Subject: Re: [PATCH] libxl: do not fail device removal if backend domain is gone
Date: Fri, 9 Feb 2018 17:32:47 +0100	[thread overview]
Message-ID: <20180209163247.GQ2070@mail-itl> (raw)
In-Reply-To: <20180209153340.2n6xg7okbgg2umyq@MacBook-Pro-de-Roger.local>

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On Fri, Feb 09, 2018 at 03:33:40PM +0000, Roger Pau Monné wrote:
> I'm sorry, I'm a little foggy today. Does this mean the call to
> libxl__xs_path_cleanup is simply not needed in
> libxl__initiate_device_generic_remove?

It is, it's an alternative to setting be/state=XenbusStateClosing, when
frontend is unresponsive. To let the backend know that frontend is gone,
so it can set be/state=XenbusStateClosed.

We have various cases (not comprehensive list):

 - both frontend and backend operational: after setting
   be/state=XenbusStateClosing backend wait for frontend confirmation
   and respond with be/state=XenbusStateClosed; then libxl in dom0
   remove frontend entries and libxl in backend domain (which may be the
   same) remove backend entries
 - unresponsive backend/frontend: after a timeout, force=1 is used to remove
   frontend entries, instead of just setting
   be/state=XenbusStateClosing; then wait for be/state=XenbusStateClosed.
   If that timeout too, remove both frontend and backend entries
 - backend gone, with this patch: no place for setting/waiting on
   be/state - go directly to removing frontend entries, without waiting
   for be/state=XenbusStateClosed (this is the difference vs force=1)

Without this patch the end result is similar, both frontend and backend
entries are removed, but in case of backend gone:
 - libxl waits for be/state=XenbusStateClosed (and obviously timeout)
 - return value from the function signal an error, which for example
   confuse libvirt - it thinks the device remove failed, so is still
   there

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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