sched: Check if lowest_mask is initialized in find_lowest_rq()

On system boot up, the lowest_mask is initialized with an
early_initcall(). But RT tasks may wake up on other
early_initcall() callers before the lowest_mask is initialized,
causing a system crash.

Commit "d72bce0e rcu: Cure load woes" was the first commit
to wake up RT tasks in early init. Before this commit this bug
should not happen.
Reported-by: Andrew Theurer <habanero@linux.vnet.ibm.com>
Tested-by: Andrew Theurer <habanero@linux.vnet.ibm.com>
Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20110614223657.824872966@goodmis.orgSigned-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched_rt.c

@@ -1239,6 +1239,10 @@ static int find_lowest_rq(struct task_struct *task)
 	int this_cpu = smp_processor_id();
 	int cpu      = task_cpu(task);
 
+	/* Make sure the mask is initialized first */
+	if (unlikely(!lowest_mask))
+		return -1;
+
 	if (task->rt.nr_cpus_allowed == 1)
 		return -1; /* No other targets possible */