[libvirt] [PATCH] libxl: find virDomainObj in libxlDomainShutdownThread

[Martin Kletzander]

On Wed, Sep 21, 2016 at 03:29:49PM -0600, Jim Fehlig wrote:
>libxl events are delivered to libvirt via the libxlDomainEventHandler
>callback registered with libxl. Documenation in
>$xensrc/tools/libxl/libxl_event.h states that the callback "may occur
>on any thread in which the application calls libxl". This can result
>in deadlock since many of the libvirt callees of libxl hold a lock on
>the virDomainObj they are working on. When the callback is invoked, it
>attempts to find a virDomainObj corresponding to the domain ID provided
>by libxl. Searching the domain obj list results in locking each obj
>before checking if it is active, and its ID equals the requested ID.
>Deadlock is possible when attempting to lock an obj that is already
>locked further up the call stack. Indeed, Max Ustermann reported an
>instance of this deadlock
>
>https://www.redhat.com/archives/libvir-list/2015-November/msg00130.html
>
>Guido Rossmueller also recently stumbled across it
>
>https://www.redhat.com/archives/libvir-list/2016-September/msg00287.html
>
>Fix the deadlock by moving the lookup of virDomainObj to the
>libxlDomainShutdownThread. After this patch, libxl events are
>enqueued on the libvirt side and processed by dedicated thread,
>avoiding the described deadlock.
>

Makes sense, passing locked object to another thread is not good, the
references match, so even though I cannot test this out, I'm fairly sure
when giving it an ACK.
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