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

[Paul E. McKenney]

On Thu, Apr 06, 2017 at 10:14:25AM -0400, Steven Rostedt wrote:
> On Wed, 5 Apr 2017 21:15:15 -0700
> "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> \> > 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);  
> > 
> > Great header comment on ftrace_sync(): "Yes, function tracing is rude."
> > And schedule_on_each_cpu() looks like a great workqueue gatling gun!  ;-)
> > 
> > > +#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  
> > 
> > How about this to save a line?
> > 
> > 		if (IS_ENABLED(CONFIG_PREEMPT))
> > 			synchronize_rcu_tasks();
> 
> Ah, this works as gcc optimizes it out. Otherwise I received a compile
> error with synchronize_rcu_tasks() not defined. But that's because I
> never enabled CONFIG_TASKS_RCU.

Should I define a synchronize_rcu_tasks() that does BUG() for this case?

> > One thing that might speed this up a bit (or might not) would be to
> > doe the schedule_on_each_cpu() from a delayed workqueue.  That way,
> > if any of the activity from schedule_on_each_cpu() involved a voluntary
> > context switch (from a cond_resched() or some such), then
> > synchronize_rcu_tasks() would get the benefit of that context switch.
> > 
> > You would need a flush_work() to wait for that delayed workqueue
> > as well, of course.
> 
> This is a very slow path, I'm not too interested in making it complex
> to speed it up.

Makes sense to me!

> > Not sure whether it is worth it, but figured I should pass it along.
> > 
> > >  		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);
> > >  }  
> > 
> > Agreed, straightforward patch!
> 
> Great, I'll start making it official then.

Sounds very good!

							Thanx, Paul