Xen devel - Aug 2011 - NetBSD and hotplug scripts synchronization using libxl

Hello,

With the introduction of libxl the deletion of all xenstore entries
regarding a domU cleanup is done in libxl itself, while in xend the
deletion of entries regarding vbd was done in the hotplug scripts.
This presents a problem in NetBSD, since there is no synchronization
between xbdback (the kernel driver in charge of disconnecting the
devices), libxl and the hotplug scripts that actually unmount the
images (called from xenbackend user-space daemon). The current flow in
NetBSD is the following, typically xbdback reads a CLOSING state from
xenbus, disconnects the image and sets the state to CLOSED. And here
there''s a race condition between libxl and the hotplug scripts called
by xenbackend. Both are watching the state of xenstore entries, and
upon detecting a CLOSED state libxl deletes the entry, probably before
xenbackend has been able to read it and unmount the image.

This is the only problem left to add NetBSD compatibility to libxl,
how does this synchronization happen in Linux? I''ve tried looking at
the hotplug scripts, but I''m not able to see any kind of
sincronization, probably because Linux doesn''t use vbd anymore in
libxl, but I think there should also be some kind of synchronization
with tapdisk device unmouting.

Thanks, Roger.

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Roger Pau Monné writes ("[Xen-devel] NetBSD and hotplug scripts
synchronization using libxl"):
> This is the only problem left to add NetBSD compatibility to libxl,
> how does this synchronization happen in Linux? I''ve tried looking
at
> the hotplug scripts, but I''m not able to see any kind of
> sincronization, probably because Linux doesn''t use vbd anymore in
> libxl, but I think there should also be some kind of synchronization
> with tapdisk device unmouting.
In libxl, libxl waits for the device to be shut down, and is then
responsible for undoing whatever was necessary to make the device
available to blkback (ie, what you are calling "unmounting").

For things which start out as block devices nothing needs to be done
since blkback can use any block device directly.

Ian.

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