[PATCH -tip] cpuacct: Make cpuacct hierarchy walk in cpuacct_charge() safe when rcupreempt is used.

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

cpuacct: Make cpuacct hierarchy walk in cpuacct_charge() safe when
	rcupreempt is used.

cpuacct_charge() obtains task's ca and does a hierarchy walk upwards.
This can race with the task's movement between cgroups. This race
can cause an access to freed ca pointer in cpuacct_charge(). This will not
happen with rcu or tree rcu as cpuacct_charge() is called with preemption
disabled. However if rcupreempt is used, the race still exists. Thanks to
Li Zefan for explaining this.

Fix this race by explicitly protecting ca and the hierarchy walk with
rcu_read_lock().

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
---
 kernel/sched.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -9891,6 +9891,13 @@ static void cpuacct_charge(struct task_s
 		return;
 
 	cpu = task_cpu(tsk);
+
+	/*
+	 * preemption is already disabled here, but to be safe with
+	 * rcupreempt, take rcu_read_lock(). This protects ca and
+	 * hence the hierarchy walk.
+	 */
+	rcu_read_lock();
 	ca = task_ca(tsk);
 
 	do {
@@ -9898,6 +9905,7 @@ static void cpuacct_charge(struct task_s
 		*cpuusage += cputime;
 		ca = ca->parent;
 	} while (ca);
+	rcu_read_unlock();
 }
 
 struct cgroup_subsys cpuacct_subsys = {