[PATCH 4.2 035/110] tracing: Have stack tracer force RCU to be watching

[Greg Kroah-Hartman]

4.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: "Steven Rostedt (Red Hat)" <rostedt@xxxxxxxxxxx>

commit a2d7629048322ae62bff57f34f5f995e25ed234c upstream.

The stack tracer was triggering the WARN_ON() in module.c:

 static void module_assert_mutex_or_preempt(void)
 {
 #ifdef CONFIG_LOCKDEP
	if (unlikely(!debug_locks))
		return;

	WARN_ON(!rcu_read_lock_sched_held() &&
		!lockdep_is_held(&module_mutex));
 #endif
 }

The reason is that the stack tracer traces all function calls, and some of
those calls happen while exiting or entering user space and idle. Some of
these functions are called after RCU had already stopped watching, as RCU
does not watch userspace or idle CPUs.

If a max stack is hit, then the save_stack_trace() is called, which will
check module addresses and call module_assert_mutex_or_preempt(), and then
trigger the warning. Sad part is, the warning itself will also do a stack
trace and tigger the same warning. That probably should be fixed.

The warning was added by 0be964be0d45 "module: Sanitize RCU usage and
locking" but this bug has probably been around longer. But it's unlikely to
cause much harm, but the new warning causes the system to lock up.

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc:"Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/trace/trace_stack.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/kernel/trace/trace_stack.c
+++ b/kernel/trace/trace_stack.c
@@ -94,6 +94,12 @@ check_stack(unsigned long ip, unsigned l
 	local_irq_save(flags);
 	arch_spin_lock(&max_stack_lock);
 
+	/*
+	 * RCU may not be watching, make it see us.
+	 * The stack trace code uses rcu_sched.
+	 */
+	rcu_irq_enter();
+
 	/* In case another CPU set the tracer_frame on us */
 	if (unlikely(!frame_size))
 		this_size -= tracer_frame;
@@ -174,6 +180,7 @@ check_stack(unsigned long ip, unsigned l
 	}
 
  out:
+	rcu_irq_exit();
 	arch_spin_unlock(&max_stack_lock);
 	local_irq_restore(flags);
 }


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