Messages in this thread | | | Date | Wed, 12 Apr 2017 11:53:04 -0400 | From | Steven Rostedt <> | Subject | Re: There is a Tasks RCU stall warning |
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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 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.
> > 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.
> > 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.
> > 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.
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();
-- Steve
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