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This was also found while investigating https://bugzilla.redhat.com/show_bug.cgi?id=670848 An EOF on a domain's monitor socket results in an event being queued to handle the EOF. The handler calls qemuProcessHandleMonitorEOF. If it is a transient domain, this leads to a call to virDomainRemoveInactive, which removes the domain from the driver's hashtable and unref's it. Nowhere in this code is the qemu driver lock acquired. However, all modifications to the driver's domain hashtable *must* be done while holding the driver lock, otherwise the hashtable can become corrupt, and (even more likely) another thread could call a different hashtable function and acquire a pointer to the domain that is in the process of being destroyed. To prevent such a disaster, qemuProcessHandleMonitorEOF must get the qemu driver lock *before* it gets the DomainObj's lock, and hold it until it is finished with the DomainObj. This guarantees that nobody else modifies the hashtable at the same time, and that anyone who had already gotten the DomainObj from the hashtable prior to this call has finished with it before we remove/destroy it.
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>