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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. Reported-by: Max Ustermann <ustermann78@web.de> Reported-by: Guido Rossmueller <Guido.Rossmueller@gdata.de>
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>