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SubjectRe: There is a Tasks RCU stall warning
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

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