Re: [libvirt PATCH 2/3] util: assign macvtap names using a monotonically increasing integer

There have been some reports that, due to libvirt always trying to assign the lowest numbered macvtap / tap device name possible, a new guest would sometimes be started using the same tap device name as previously used by another guest that is in the process of being destroyed *as the new guest is starting.

In some cases this has led to, for example, the old guest's qemuProcessStop() code deleting a port from an OVS switch that had just been re-added by the new guest (because the port name is based on only the device name using the port). Similar problems can happen (and I believe have) with nwfilter rules and bandwidth rules (which are both instantiated based on the name of the tap device). A couple patches have been previously proposed to change the ordering of startup and shutdown processing, or to put a mutex around everything related to the tap/macvtap device name usage, but in the end no matter what you do there will still be possible holes, because the device could be deleted outside libvirt's control (for example, regular tap devices are automatically deleted when the qemu process terminates, and that isn't always initiated by libvirt but could instead happen completely asynchronously - libvirt then has no control over the ordering of shutdown operations, and no opportunity to protect it with a mutex.) But this only happens if a new device is created at the same time as one is being deleted. We can effectively eliminate the chance of this happening if we end the practice of always looking for the lowest numbered available device name, and instead just keep an integer that is incremented each time we need a new device name. At some point it will need to wrap back around to 0 (in order to avoid the IFNAMSIZ 15 character limit if nothing else), and we can't guarantee that the new name really will be the *least* recently used name, but "math" suggests that it will be *much* less common that we'll try to re-use the *most* recently used name. This patch implements such a counter for macvtap/macvlan only. It does it on top of the existing "ID reservation" system (I'm thinking about making a followup that gets rid of the bitmap, as it's now overkill and just serves to make the code more confusing). Signed-off-by: Laine Stump <laine(a)redhat.com&gt; --- src/util/virnetdevmacvlan.c | 67 +++++++++++++++++++++++++++---------- 1 file changed, 50 insertions(+), 17 deletions(-)