[PATCH] introduce idle_task_exit

Heiko Carstens figured out that offlining a cpu can leak mm_structs because
the dying cpu's idle task fails to switch to init_mm and mmdrop its
active_mm before the cpu is down.  This patch introduces idle_task_exit,
which allows the idle task to do this as Ingo suggested.

I will follow this up with a patch for ppc64 which calls idle_task_exit
from cpu_die.
Signed-off-by: Nathan Lynch <nathanl@austin.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/sched.h

@@ -756,6 +756,7 @@ extern void sched_exec(void);
 #endif
 
 extern void sched_idle_next(void);
+extern void idle_task_exit(void);
 extern void set_user_nice(task_t *p, long nice);
 extern int task_prio(const task_t *p);
 extern int task_nice(const task_t *p);

kernel/sched.c

@@ -3976,6 +3976,20 @@ void sched_idle_next(void)
 	spin_unlock_irqrestore(&rq->lock, flags);
 }
 
+/* Ensures that the idle task is using init_mm right before its cpu goes
+ * offline.
+ */
+void idle_task_exit(void)
+{
+	struct mm_struct *mm = current->active_mm;
+
+	BUG_ON(cpu_online(smp_processor_id()));
+
+	if (mm != &init_mm)
+		switch_mm(mm, &init_mm, current);
+	mmdrop(mm);
+}
+
 static void migrate_dead(unsigned int dead_cpu, task_t *tsk)
 {
 	struct runqueue *rq = cpu_rq(dead_cpu);