Re: [PATCH] network: Fix a race condition when shutdown & start vm at the same time

On 6/19/20 12:26 PM, Daniel P. Berrangé wrote: Yeah, that's what we do for macvtap. It is extra code, non-trivial, and therefore prone to errors (as in, "anything over 1 line long is prone to errors"). I *thought* I had remembered that couldn't work properly for standard tap devices couldn't work due to the way that tap devices are "created" (you first open /dev/net/tun, then issue a ioctl(TUNSETIFF, "desiredname")  on the handle returned by open. I had been concerned that if you did that with an existing name it would just connect you to the existing tap, but I tried it just now and got an error the 2nd time I tried to use the same tap device name, so perhaps/probably I'm again misremembering. 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 :-) 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. Note that since macvtap and standard tap devices (and, actually *all* network devices) in the same namespace need to not have conflicting names, we could keep a single list of in-use names. Since we would never auto-generate a tap device name that conflicts with an auto-generated macvtap name, that is probably unnecessary, but just in case it makes things easier... 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". In the end, if we can do something simple that preserves current behavior while covering the hole, that would probably be more expedient, but I have to say the thought of naming the devices ourselves has crossed my mind.