Expand description of goroutine+netns problems

pkg/ns/README.md

@@ -9,7 +9,7 @@ Go provides the `runtime.LockOSThread()` function to ensure a specific goroutine
 For example, you cannot rely on the `ns.Set()` namespace being the current namespace after the `Set()` call unless you do two things.  First, the goroutine calling `Set()` must have previously called `LockOSThread()`.  Second, you must ensure `runtime.UnlockOSThread()` is not called somewhere in-between.  You also cannot rely on the initial network namespace remaining the current network namespace if any other code in your program switches namespaces, unless you have already called `LockOSThread()` in that goroutine.  Note that `LockOSThread()` prevents the Go scheduler from optimally scheduling goroutines for best performance, so `LockOSThread()` should only be used in small, isolated goroutines that release the lock quickly.
 
 ### Do() The Recommended Thing
-The `ns.Do()` method provides control over network namespaces for you by implementing these strategies. All code dependent on a particular network namespace should be wrapped in the `ns.Do()` method to ensure the correct namespace is selected for the duration of your code.  For example:
+The `ns.Do()` method provides control over network namespaces for you by implementing these strategies. All code dependent on a particular network namespace (including the root namespace) should be wrapped in the `ns.Do()` method to ensure the correct namespace is selected for the duration of your code.  For example:
 
 ```go
 targetNs, err := ns.NewNS()
@@ -26,6 +26,9 @@ err = targetNs.Do(func(hostNs ns.NetNS) error {
 })
 ```
 
+Note this requirement to wrap every network call is very onerous - any libraries you call might call out to network services such as DNS, and all such calls need to be protected after you call `ns.Do()`. The CNI plugins all exit very soon after calling `ns.Do()` which helps to minimize the problem.
+
 ### Further Reading
  - https://github.com/golang/go/wiki/LockOSThread
  - http://morsmachine.dk/go-scheduler
+ - https://github.com/containernetworking/cni/issues/262