tracing/ftrace: Fix a race condition in sched_switch tracer

(depends on Tracing/fastboot: Enable boot tracing only during initcalls)
---
This patch fixes a race condition in the sched_switch tracer. If several tasks (IE: concurrent initcalls) are playing with tracing_start_cmdline_record() and tracing_stop_cmdline_record(), the following situation could happen:

_ Task A and B are using the same tracepoint probe. Task A holds it. Task B is sleeping and doesn't hold it.
_ Task A frees the sched tracer, then sched_ref is decremented to 0. _ Task A is preempted and hadn't yet unregistered its tracepoint probe, then B runs.
_ B increments sched_ref, sees it's 1 and then guess it has to register its probe. But it has not been freed by task A.
_ A lot of bad things can happen after that...

---
 kernel/trace/trace_sched_switch.c |    5 +++++
 1 files changed, 5 insertions(+), 0 deletions(-)

index b8f56be..59de514 100644
@@ -17,6 +17,7 @@
 static struct trace_array	*ctx_trace;
 static int __read_mostly	tracer_enabled;
 static atomic_t			sched_ref;
+static DEFINE_MUTEX(tracepoint_mutex);
 
 static void
 probe_sched_switch(struct rq *__rq, struct task_struct *prev,

@@ -125,18 +126,22 @@ static void tracing_start_sched_switch(void)
 {
 	long ref;
 
+	mutex_lock(&tracepoint_mutex);
 	ref = atomic_inc_return(&sched_ref);
 	if (ref == 1)
 		tracing_sched_register();
+	mutex_unlock(&tracepoint_mutex);
 }
 
 static void tracing_stop_sched_switch(void)
 {
 	long ref;
 
+	mutex_lock(&tracepoint_mutex);
 	ref = atomic_dec_and_test(&sched_ref);
 	if (ref)
 		tracing_sched_unregister();
+	mutex_unlock(&tracepoint_mutex);
 }
 
 void tracing_start_cmdline_record(void)