[tip:sched/urgent] sched: Check if lowest_mask is initialized in find_lowest_rq()

[tip-bot for Steven Rostedt]

Commit-ID:  0da938c44921cfb690283d3b0c9c48a10375db2c
Gitweb:     http://git.kernel.org/tip/0da938c44921cfb690283d3b0c9c48a10375db2c
Author:     Steven Rostedt <rostedt@xxxxxxxxxxx>
AuthorDate: Tue, 14 Jun 2011 18:36:25 -0400
Committer:  Ingo Molnar <mingo@xxxxxxx>
CommitDate: Wed, 15 Jun 2011 11:44:48 +0200

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 "d72bce0e67 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@xxxxxxxxxxxxxxxxxx>
Tested-by: Andrew Theurer <habanero@xxxxxxxxxxxxxxxxxx>
Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Acked-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20110614223657.824872966@xxxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
 kernel/sched_rt.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
index 9b8d5dc..10d0182 100644
--- a/kernel/sched_rt.c
+++ b/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 */
 
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