Fix scheduler deadlock on some platforms.

Some platforms need to grab mm->page_table_lock during switch_mm().
On the other hand code like swap_out() in mm/vmscan.c needs to hold
mm->page_table_lock during wakeups which needs to grab the runqueue
lock.  This creates a conflict and the resolution chosen here is to
not hold the runqueue lock during context_switch().

The implementation is specifically a "frozen" state implemented as a
spinlock, which is held around the context_switch() call.  This allows
the runqueue lock to be dropped during this time yet prevent another cpu
from running the "not switched away from yet" task.

kernel/sched.c

@@ -140,6 +140,7 @@ struct prio_array {
  */
 struct runqueue {
 	spinlock_t lock;
+	spinlock_t frozen;
 	unsigned long nr_running, nr_switches, expired_timestamp;
 	task_t *curr, *idle;
 	prio_array_t *active, *expired, arrays[2];
@@ -400,7 +401,7 @@ void sched_exit(task_t * p)
 #if CONFIG_SMP || CONFIG_PREEMPT
 asmlinkage void schedule_tail(void)
 {
-	spin_unlock_irq(&this_rq()->lock);
+	spin_unlock_irq(&this_rq()->frozen);
 }
 #endif
 
@@ -808,16 +809,22 @@ asmlinkage void schedule(void)
 	if (likely(prev != next)) {
 		rq->nr_switches++;
 		rq->curr = next;
+		spin_lock(&rq->frozen);
+		spin_unlock(&rq->lock);
+
 		context_switch(prev, next);
+
 		/*
 		 * The runqueue pointer might be from another CPU
 		 * if the new task was last running on a different
 		 * CPU - thus re-load it.
 		 */
-		barrier();
+		mb();
 		rq = this_rq();
+		spin_unlock_irq(&rq->frozen);
+	} else {
+		spin_unlock_irq(&rq->lock);
 	}
-	spin_unlock_irq(&rq->lock);
 
 	reacquire_kernel_lock(current);
 	preempt_enable_no_resched();
@@ -1463,6 +1470,7 @@ void __init sched_init(void)
 		rq->active = rq->arrays;
 		rq->expired = rq->arrays + 1;
 		spin_lock_init(&rq->lock);
+		spin_lock_init(&rq->frozen);
 		INIT_LIST_HEAD(&rq->migration_queue);
 
 		for (j = 0; j < 2; j++) {