pid: Improve the comment about waiting in zap_pid_ns_processes

Oleg wrote a very informative comment, but with the removal of
proc_cleanup_work it is no longer accurate.

Rewrite the comment so that it only talks about the details
that are still relevant, and hopefully is a little clearer.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index 318fcc6..01f8ba3 100644 (file)
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -224,20 +224,27 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
        } while (rc != -ECHILD);
 
        /*
-        * kernel_wait4() above can't reap the EXIT_DEAD children but we do not
-        * really care, we could reparent them to the global init. We could
-        * exit and reap ->child_reaper even if it is not the last thread in
-        * this pid_ns, free_pid(pid_allocated == 0) calls proc_cleanup_work(),
-        * pid_ns can not go away until proc_kill_sb() drops the reference.
+        * kernel_wait4() misses EXIT_DEAD children, and EXIT_ZOMBIE
+        * process whose parents processes are outside of the pid
+        * namespace.  Such processes are created with setns()+fork().
         *
-        * But this ns can also have other tasks injected by setns()+fork().
-        * Again, ignoring the user visible semantics we do not really need
-        * to wait until they are all reaped, but they can be reparented to
-        * us and thus we need to ensure that pid->child_reaper stays valid
-        * until they all go away. See free_pid()->wake_up_process().
+        * If those EXIT_ZOMBIE processes are not reaped by their
+        * parents before their parents exit, they will be reparented
+        * to pid_ns->child_reaper.  Thus pidns->child_reaper needs to
+        * stay valid until they all go away.
         *
-        * We rely on ignored SIGCHLD, an injected zombie must be autoreaped
-        * if reparented.
+        * The code relies on the the pid_ns->child_reaper ignoring
+        * SIGCHILD to cause those EXIT_ZOMBIE processes to be
+        * autoreaped if reparented.
+        *
+        * Semantically it is also desirable to wait for EXIT_ZOMBIE
+        * processes before allowing the child_reaper to be reaped, as
+        * that gives the invariant that when the init process of a
+        * pid namespace is reaped all of the processes in the pid
+        * namespace are gone.
+        *
+        * Once all of the other tasks are gone from the pid_namespace
+        * free_pid() will awaken this task.
         */
        for (;;) {
                set_current_state(TASK_INTERRUPTIBLE);