Patch "tracing: Have stack tracer force RCU to be watching" has been added to the 4.2-stable tree

This is a note to let you know that I've just added the patch titled

tracing: Have stack tracer force RCU to be watching

to the 4.2-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
tracing-have-stack-tracer-force-rcu-to-be-watching.patch
and it can be found in the queue-4.2 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Steven Rostedt (Red Hat)" <[email protected]>
Date: Tue, 20 Oct 2015 11:38:08 -0400
Subject: tracing: Have stack tracer force RCU to be watching

From: "Steven Rostedt (Red Hat)" <[email protected]>

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.

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

@@ -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);
 }

Patches currently in stable-queue which might be from [email protected] are

queue-4.2/tracing-have-stack-tracer-force-rcu-to-be-watching.patch