tracing/wakeup: move access to wakeup_cpu into spinlock

The code had the following outside the lock:

        if (next != wakeup_task)
                return;

        pc = preempt_count();

        /* The task we are waiting for is waking up */
        data = wakeup_trace->data[wakeup_cpu];

On initialization, wakeup_task is NULL and wakeup_cpu -1. This code
is not under a lock. If wakeup_task is set on another CPU as that
task is waking up, we can see the wakeup_task before wakeup_cpu is
set. If we read wakeup_cpu while it is still -1 then we will have
a bad data pointer.

This patch moves the reading of wakeup_cpu within the protection of
the spinlock used to protect the writing of wakeup_cpu and wakeup_task.

[ Impact: remove possible race causing invalid pointer dereference ]

Reported-by: Maneesh Soni <maneesh@in.ibm.com>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>

kernel/trace/trace_sched_wakeup.c

@@ -138,9 +138,6 @@ probe_wakeup_sched_switch(struct rq *rq, struct task_struct *prev,
 
 	pc = preempt_count();
 
-	/* The task we are waiting for is waking up */
-	data = wakeup_trace->data[wakeup_cpu];
-
 	/* disable local data, not wakeup_cpu data */
 	cpu = raw_smp_processor_id();
 	disabled = atomic_inc_return(&wakeup_trace->data[cpu]->disabled);
@@ -154,6 +151,9 @@ probe_wakeup_sched_switch(struct rq *rq, struct task_struct *prev,
 	if (unlikely(!tracer_enabled || next != wakeup_task))
 		goto out_unlock;
 
+	/* The task we are waiting for is waking up */
+	data = wakeup_trace->data[wakeup_cpu];
+
 	trace_function(wakeup_trace, CALLER_ADDR0, CALLER_ADDR1, flags, pc);
 	tracing_sched_switch_trace(wakeup_trace, prev, next, flags, pc);