On Thu, 2012-04-26 at 16:59 -0700, Andrew Morton wrote:

quoted

+void clear_tasks_mm_cpumask(int cpu)

The operation of this function was presumably obvious to you at the
time you wrote it, but that isn't true of other people at later times.

Please document it?

quoted

+{
+     struct task_struct *p;
+
+     /*
+      * This function is called after the cpu is taken down and marked
+      * offline,

hm, well.  Who said that this function will only ever be called
after that CPU was taken down?  There is nothing in the function name
nor in the (absent) documentation which enforces this precondition.

If someone tries to use this function for a different purpose, or
copies-and-modifies it for a different purpose, we just shot them in
the foot.

They'd be pretty dumb to do that without reading the local comment,
but still...

Methinks something simple like:

	WARN_ON(cpu_online(cpu));

Ought to cure that worry, no? :-)

quoted

       so its not like new tasks will ever get this cpu set in
+      * their mm mask. -- Peter Zijlstra
+      * Thus, we may use rcu_read_lock() here, instead of grabbing
+      * full-fledged tasklist_lock.
+      */
+     rcu_read_lock();
+     for_each_process(p) {
+             struct task_struct *t;
+
+             t = find_lock_task_mm(p);
+             if (!t)
+                     continue;
+             cpumask_clear_cpu(cpu, mm_cpumask(t->mm));
+             task_unlock(t);
+     }
+     rcu_read_unlock();
+}