Re: [libvirt] RFC: libxl race fixes

Jim Fehlig wrote: > I've been investigating some races in the libxl driver and would like to > get comments on some potential solutions. > > The first race is in the fd/timeout event handling code, which maps > libxl's osevent interface to libvirt's event loop interface. This > mapping opens the possibility for libvirt's event loop to invoke > event callbacks after libxl has already deregistered the event, > potentially accessing an event object that has already been freed. > > One solution to this race I've found successful is reference counting the > objects associated with the events. When libxl registers an event, an > object encapsulating the event is created and it's reference count is set > to 1. When the event is injected into libvirt's event loop, another > reference is taken on the object. When libxl deregisters the event, it's > reference count is decremented. Once the event is removed from libvirt's > event loop, the final reference is decremented and the object is disposed. > While rebasing this solution to use danpb's recent virObjectLockable change, I revisited using only a lock in the libxlDomainObject, acquiring the lock in the registration, deregistration, modify, and callback functions. I recall some problems with this approach before, but now find it to be sufficient. Perhaps a misplaced lock drove me to the unneeded complexity of this double lock nonsense... > This approach ensures the object is not disposed until it is removed > from libvirt's event loop *and* libxl had explicitly deregistered the event. > The notion of an event being 'disabled' found in libvirt's event loop impl > also had to be added to the libxl event object, to ensure the driver doesn't > call into libxl for a previously deregistered event. > > The second race is between destroying a vm (i.e., calling > privateDataFreeFunc, which frees the libxl ctx) and deregistration/cleanup > of all events that have been registered by libxl. > > One solution for this race is to convert libxlDomainObjPrivate to a > virObject and increment its reference for each fd and timeout registration. > Only change here is using the new virObjectLockable.