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If an async job run on a domain will stop the domain at the end of the job, a concurrently run query job can hang in qemu monitor and nothing can be done with that domain from this point on. An attempt to start such domain results in "Timed out during operation: cannot acquire state change lock" error. However, quite a few things have to happen at the right time... There must be an async job running which stops a domain at the end. This race was reported with dump --crash but other similar jobs, such as (managed)save and migration, should be able to trigger this bug as well. While this async job is processing its last monitor command, that is a query-migrate to which qemu replies with status "completed", a new libvirt API that results in a query job must arrive and stay waiting until the query-migrate command finishes. Once query-migrate is done but before the async job closes qemu monitor while stopping the domain, the other thread needs to wake up and call qemuMonitorSend to send its command to qemu. Before qemu gets a chance to respond to this command, the async job needs to close the monitor. At this point, the query job thread is waiting for a condition that no-one will ever signal so it never finishes the job.
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>