On 6/19/20 1:16 PM, Daniel P. Berrangé wrote:
On Fri, Jun 19, 2020 at 01:06:33PM -0400, Laine Stump wrote:
On 6/19/20 12:26 PM, Daniel P. Berrangé wrote:
Maintain a global "int nextTapID" counter, and just iterate on this. NIC names can be upto 16 bytes, so we'll create billions of devices before we have any chance of wrapping around and risking a collision with a concurrently shutting down guest.
I remember I tried doing a monotonically increasing "next" counter for macvtap (never actually pushed, but shown to [some users, I forget who, maybe ovirt devs?], and they didn't like it because the devices ended up with long difficult-for-a-human-to-remember/pick-out/recite names like macvtap42301 and macvtap43021. So instead we keep track of the names that are in use, and always look for the lowest available number when creating a new one. (of course doing that would greatly increase the likelyhood of exposing any race conditions, so...) Definitely if we change the behavior in this way we're going to hear about it, though :-) People might complain, but I'm not convinced it really matters. Pid numbers used to be nice & short until someone raised pid max and now my processes have 7-digit long pids. It is surprising at first, but it didn't really cause me any functional problems.
This is a good point. But even more convincing than that is the revelation that this isn't an issue just with OVS switch ports - the same race is also causing problems with libvirt nwfilter driver bindings, as reported here: https://bugzilla.redhat.com/show_bug.cgi?id=1837395 After seeing that report and realizing that this same race is the cause, I've decided to change libvirt's tap device creation to provide names based on a monotonically increasing counter as you suggest, rather than relying on the kernel. I'm hoping to have something to send to the list in a day or two.
And there's still the option of just providing a fixed name if you really need something predictable for a guest.
Another issue I remember from macvtap is the possibility of a different process (not libvirt) having already created a macvtap device with the name that we are wanting to use. That is easily solved in the case of macvtap by just iterating through until we find one that we can create without failure. So basically you have the list/bitmap/whatever of devicenames currently in use, use that as a starting point to pick a name for the new device, but then also allow for that name to already be used, and retry. I wasn't thinking we need a bitmap, literally just a single counter. We start where we left off, trying until we succeeed, which is what the kernel does anyway IIRC. Most other competing processes will rely on the kernel auto-allocation so won't clash with us very frequently if at all.
Also, in the name of backward compatibility, we will need to support tap device names from the config that have %d in the name, e.g. the default name we send is "vnet%d", but we allow for someone to specify "mytap%d". We could keep a counter per prefix, or just have a single global counter.
Regards, Daniel