1 files changed, 45 insertions, 0 deletions

diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt
index be675d2d15a7..a0b005d2bd95 100644
--- a/Documentation/cpu-hotplug.txt
+++ b/Documentation/cpu-hotplug.txt
@@ -312,12 +312,57 @@ things will happen if a notifier in path sent a BAD notify code.
 Q: I don't see my action being called for all CPUs already up and running?
 A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.
    If you need to perform some action for each cpu already in the system, then
+   do this:
 
         for_each_online_cpu(i) {
                 foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);
                 foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i);
         }
 
+   However, if you want to register a hotplug callback, as well as perform
+   some initialization for CPUs that are already online, then do this:
+
+   Version 1: (Correct)
+   ---------
+
+        cpu_notifier_register_begin();
+
+                for_each_online_cpu(i) {
+                        foobar_cpu_callback(&foobar_cpu_notifier,
+                                            CPU_UP_PREPARE, i);
+                        foobar_cpu_callback(&foobar_cpu_notifier,
+                                            CPU_ONLINE, i);
+                }
+
+        /* Note the use of the double underscored version of the API */
+        __register_cpu_notifier(&foobar_cpu_notifier);
+
+        cpu_notifier_register_done();
+
+   Note that the following code is *NOT* the right way to achieve this,
+   because it is prone to an ABBA deadlock between the cpu_add_remove_lock
+   and the cpu_hotplug.lock.
+
+   Version 2: (Wrong!)
+   ---------
+
+        get_online_cpus();
+
+                for_each_online_cpu(i) {
+                        foobar_cpu_callback(&foobar_cpu_notifier,
+                                            CPU_UP_PREPARE, i);
+                        foobar_cpu_callback(&foobar_cpu_notifier,
+                                            CPU_ONLINE, i);
+                }
+
+        register_cpu_notifier(&foobar_cpu_notifier);
+
+        put_online_cpus();
+
+    So always use the first version shown above when you want to register
+    callbacks as well as initialize the already online CPUs.
+
+
 Q: If i would like to develop cpu hotplug support for a new architecture,
    what do i need at a minimum?
 A: The following are what is required for CPU hotplug infrastructure to work