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The meat of this patch is just moving the calls to virNWFilterRegisterCallbackDriver from each hypervisor's "register" function into its "initialize" function. The rest is just code movement to allow that, and a new virNWFilterUnRegisterCallbackDriver function to undo what the register function does. The long explanation: There is an array in nwfilter called callbackDrvArray that has pointers to a table of functions for each hypervisor driver that are called by nwfilter. One of those function pointers is to a function that will lock the hypervisor driver. Entries are added to the table by calling each driver's "register" function, which happens quite early in libvirtd's startup. Sometime later, each driver's "initialize" function is called. This function allocates a driver object and stores a pointer to it in a static variable that was previously initialized to NULL. (and here's the important part...) If the "initialize" function fails, the driver object is freed, and that pointer set back to NULL (but the entry in nwfilter's callbackDrvArray is still there). When the "lock the driver" function mentioned above is called, it assumes that the driver was successfully loaded, so it blindly tries to call virMutexLock on "driver->lock". BUT, if the initialize never happened, or if it failed, "driver" is NULL. And it just happens that "lock" is always the first field in driver so it is also NULL. Boom. To fix this, the call to virNWFilterRegisterCallbackDriver for each driver shouldn't be called until the end of its (*already guaranteed successful*) "initialize" function, not during its "register" function (which is currently the case). This implies that there should also be a virNWFilterUnregisterCallbackDriver() function that is called in a driver's "shutdown" function (although in practice, that function is currently never called).
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>