Re: There is a Tasks RCU stall warning

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Apr 12, 2017 at 11:53:04AM -0400, Steven Rostedt wrote:
> On Wed, 12 Apr 2017 08:18:17 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> 
> 
> > > Well the trampolines pretty much can, but they are removed before
> > > calling synchronize_rcu_tasks(), and nothing can enter the trampoline
> > > when that is called.  
> > 
> > Color me confused...
> > 
> > So you can have an arbitrary function call within a trampoline?
> 
> Sorta.
> 
> When you do register_ftrace_function(ops), where ops has ops->func that
> points to a function you want to have called when a function is traced,
> the following happens (if there's no other ops registered). Let's use
> an example where ops is filtered on just the schedule() function call:
> 
> 
>  <schedule>:
>     call trampoline ---+
>     [..]               |
>                        +--> <trampoline>:
>                               push regs
>                               call ops->func
>                               pop regs
>                               ret
> 
> But that ops->func() must be very limited in what it can do. Although,
> it may actually call an rcu_read_lock()! But if that's the case, it
> must either check if rcu is watching (which perf does), or enable rcu
> via the rcu_irq_enter() with a check on rcu_irq_enter_disabled(), which
> my stack tracer does.
> 
> Now this can be called even from NMI context! Thus what ops->func does
> must be aware of that. The stack tracer func has an:
> 
>   if (in_nmi())
> 	return;
> 
> Because it needs to grab spin locks.

But preemption is enabled within the trampoline?  If so, then if
CONFIG_RCU_BOOST is set, rcu_read_unlock() can call rt_mutex_unlock().
Which looks OK to me, but I thought I should mention it.

> But one thing an op->func() is never allowed to do, is to call
> schedule() directly, or even a cond_resched(). It may be preempted if
> preemption was enabled when the trampoline was hit, but it must not
> assume that it can do a voluntary schedule. That would break the
> rcu_tasks as well if it did.

OK, so it can call functions, but it is not permitted to call functions
that voluntarily block.  That should work.  (Fingers firmly crossed.)

> > If not, agreed, no problem.  Otherwise, it seems like we have a big
> > problem remaining.  Unless the functions called from a trampoline are
> > guaranteed never to do a context switch.
> 
> Well, they can be preempted, but they should never do a voluntary
> schedule. If they did, that would be bad.

OK, feeeling better now.  ;-)

> > So what exactly is the trampoline code allowed to do?  ;-)
> 
> Well, it must be able to work in an NMI context, or bail otherwise. And
> it should never schedule on its own.

Good.

> > My problem is that I have no idea what can and cannot be included in
> > trampoline code.  In absence of that information, my RCU-honed reflexes
> > jump immediately to the worst case that I can think of.  ;-)
> 
> Lets just say that it can't voluntarily sleep. Would that be good
> enough? If someday in the future I decide to let it do so, I would add
> a flag and force that ops not to be able to use a dynamic trampoline.

That would work.  Again, feeling much better now.  ;-)

> Currently, without the synchronize_rcu_tasks(), when a dynamic ops is
> registered, the functions will point to a non dynamic trampoline. That
> is, one that is never freed. It simply does:
> 
> 	preempt_disable_notrace();
> 
> 	do_for_each_ftrace_op(op, ftrace_ops_list) {
> 		/*
> 		 * Check the following for each ops before calling their func:
> 		 *  if RCU flag is set, then rcu_is_watching() must be true
> 		 *  if PER_CPU is set, then ftrace_function_local_disable()
> 		 *                          must be false
> 		 *  Otherwise test if the ip matches the ops filter
> 		 *
> 		 * If any of the above fails then the op->func() is not executed.
> 		 */
> 		if ((!(op->flags & FTRACE_OPS_FL_RCU) || rcu_is_watching()) &&
> 		    (!(op->flags & FTRACE_OPS_FL_PER_CPU) ||
> 		     !ftrace_function_local_disabled(op)) &&
> 		    ftrace_ops_test(op, ip, regs)) {
> 		    
> 			if (FTRACE_WARN_ON(!op->func)) {
> 				pr_warn("op=%p %pS\n", op, op);
> 				goto out;
> 			}
> 			op->func(ip, parent_ip, op, regs);
> 		}
> 	} while_for_each_ftrace_op(op);
> out:
> 	preempt_enable_notrace();

Makes sense!

							Thanx, Paul