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

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.