Messages in this thread | | | Date | Wed, 12 Apr 2017 09:26:10 -0700 | From | "Paul E. McKenney" <> | Subject | Re: There is a Tasks RCU stall warning |
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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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