tracing: Make sure RCU is watching before calling a stack trace

void __trace_stack(struct trace_array *tr, unsigned long flags, int skip,

		   int pc)

{

	__ftrace_trace_stack(tr->trace_buffer.buffer, flags, skip, pc, NULL);

	struct ring_buffer *buffer = tr->trace_buffer.buffer;

	if (rcu_is_watching()) {

		__ftrace_trace_stack(buffer, flags, skip, pc, NULL);

		return;

	}

	/*

	 * When an NMI triggers, RCU is enabled via rcu_nmi_enter(),

	 * but if the above rcu_is_watching() failed, then the NMI

	 * triggered someplace critical, and rcu_irq_enter() should

	 * not be called from NMI.

	 */

	if (unlikely(in_nmi()))

		return;

	/*

	 * It is possible that a function is being traced in a

	 * location that RCU is not watching. A call to

	 * rcu_irq_enter() will make sure that it is, but there's

	 * a few internal rcu functions that could be traced

	 * where that wont work either. In those cases, we just

	 * do nothing.

	 */

	if (unlikely(rcu_irq_enter_disabled()))

		return;

	rcu_irq_enter_irqson();

	__ftrace_trace_stack(buffer, flags, skip, pc, NULL);

	rcu_irq_exit_irqson();

}

/**