Re: [BUG] stack tracing causes: kernel/module.c:271 module_assert_mutex_or_preempt

[Steven Rostedt]

On Wed, 5 Apr 2017 10:59:25 -0700
"Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

> > > Could you please let me know if tracing happens in NMI handlers?
> > > If so, a bit of additional code will be needed.
> > > 
> > > 							Thanx, Paul
> > > 
> > > PS.  Which reminds me, any short-term uses of RCU_TASKS?  This represents
> > >      3 of my 16 test scenarios, which is getting hard to justify for
> > >      something that isn't used.  Especially given that I will need to
> > >      add more scenarios for parallel-callbacks SRCU...  
> > 
> > The RCU_TASK implementation is next on my todo list. Yes, there's going
> > to be plenty of users very soon. Not for 4.12 but definitely for 4.13.
> > 
> > Sorry for the delay in implementing that :-/  
> 
> OK, I will wait a few months before checking again...
> 

Actually, I took a quick look at what needs to be done, and I think it
is *really* easy, and may be available in 4.12! Here's the current
patch.

I can probably do a patch to allow optimized kprobes on PREEMPT kernels
as well.

-- Steve

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 8efd9fe..28e3019 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -2808,18 +2808,28 @@ static int ftrace_shutdown(struct ftrace_ops *ops, int command)
 	 * callers are done before leaving this function.
 	 * The same goes for freeing the per_cpu data of the per_cpu
 	 * ops.
-	 *
-	 * Again, normal synchronize_sched() is not good enough.
-	 * We need to do a hard force of sched synchronization.
-	 * This is because we use preempt_disable() to do RCU, but
-	 * the function tracers can be called where RCU is not watching
-	 * (like before user_exit()). We can not rely on the RCU
-	 * infrastructure to do the synchronization, thus we must do it
-	 * ourselves.
 	 */
 	if (ops->flags & (FTRACE_OPS_FL_DYNAMIC | FTRACE_OPS_FL_PER_CPU)) {
+		/*
+		 * We need to do a hard force of sched synchronization.
+		 * This is because we use preempt_disable() to do RCU, but
+		 * the function tracers can be called where RCU is not watching
+		 * (like before user_exit()). We can not rely on the RCU
+		 * infrastructure to do the synchronization, thus we must do it
+		 * ourselves.
+		 */
 		schedule_on_each_cpu(ftrace_sync);
 
+#ifdef CONFIG_PREEMPT
+		/*
+		 * When the kernel is preeptive, tasks can be preempted
+		 * while on a ftrace trampoline. Just scheduling a task on
+		 * a CPU is not good enough to flush them. Calling
+		 * synchronize_rcu_tasks() will wait for those tasks to
+		 * execute and either schedule voluntarily or enter user space.
+		 */
+		synchronize_rcu_tasks();
+#endif
 		arch_ftrace_trampoline_free(ops);
 
 		if (ops->flags & FTRACE_OPS_FL_PER_CPU)
@@ -5366,22 +5376,6 @@ void __weak arch_ftrace_update_trampoline(struct ftrace_ops *ops)
 
 static void ftrace_update_trampoline(struct ftrace_ops *ops)
 {
-
-/*
- * Currently there's no safe way to free a trampoline when the kernel
- * is configured with PREEMPT. That is because a task could be preempted
- * when it jumped to the trampoline, it may be preempted for a long time
- * depending on the system load, and currently there's no way to know
- * when it will be off the trampoline. If the trampoline is freed
- * too early, when the task runs again, it will be executing on freed
- * memory and crash.
- */
-#ifdef CONFIG_PREEMPT
-	/* Currently, only non dynamic ops can have a trampoline */
-	if (ops->flags & FTRACE_OPS_FL_DYNAMIC)
-		return;
-#endif
-
 	arch_ftrace_update_trampoline(ops);
 }