Christian Brauner [off-list ref]:

Currently namespace file handles allow much broader access to namespaces
than what is currently possible via (1)-(4). The reason is that

There is no any (4) here.

On current kernels a namespace is visible to userspace in the
following cases:

[...]

(3) The namespace is a hierarchical namespace type and is the parent of
    a single or multiple child namespaces.

[...]

To handle this nicely we introduce an active reference count which
tracks (1)-(3). This is easy to do as all of these things are already

[...]

+ * Inactive -> Active:
+ *   When walking a hierarchical namespace tree upwards and reopening
+ *   parent namespaces via NS_GET_PARENT that only exist because they
+ *   are a parent of an actively used namespace it is possible to
+ *   necrobump an inactive namespace back to the active state.

These quoted parts contradict to each other. You say "we introduce an
active reference count which tracks (1)-(3)", and (3) says "The namespace
is a hierarchical namespace type and is the parent of a single or multiple
child namespaces". I. e. active reference will count such parents. But then
in code you say:

+ * Inactive -> Active:
+ *   When walking a hierarchical namespace tree upwards and reopening
+ *   parent namespaces via NS_GET_PARENT that only exist because they
+ *   are a parent of an actively used namespace it is possible to
+ *   necrobump an inactive namespace back to the active state.

I. e. now you say that such parents are inactive and can become active.




-- 
Askar Safin