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

Laine Stump laine at redhat.com
Thu Jun 25 03:06:44 UTC 2020

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:


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

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