On Wed, Aug 21, 2019 at 11:26:38AM -0400, Joel Fernandes wrote: > On Wed, Aug 21, 2019 at 10:56:17AM -0400, Joel Fernandes wrote: > > On Wed, Aug 21, 2019 at 10:38:41AM -0400, Joel Fernandes wrote: > > > On Mon, Aug 19, 2019 at 08:41:43AM -0700, Paul E. McKenney wrote: > > > > On Mon, Aug 19, 2019 at 07:33:14AM -0700, Paul E. McKenney wrote: > > > > > On Mon, Aug 19, 2019 at 05:57:57AM -0700, Paul E. McKenney wrote: > > > > > > On Sun, Aug 18, 2019 at 07:29:27PM -0700, Paul E. McKenney wrote: > > > > > > > On Sun, Aug 18, 2019 at 09:46:23PM -0400, Joel Fernandes wrote: > > > > > > > > On Sun, Aug 18, 2019 at 09:41:43PM -0400, Joel Fernandes wrote: > > > > > > > > > On Sun, Aug 18, 2019 at 06:21:53PM -0700, Paul E. McKenney wrote: > > > > > > > > [snip] > > > > > > > > > > > > Also, your commit log's point #2 is "in_irq() implies in_interrupt() > > > > > > > > > > > > which implies raising softirq will not do any wake ups." This mention > > > > > > > > > > > > of softirq seems a bit odd, given that we are going to wake up a rcuc > > > > > > > > > > > > kthread. Of course, this did nothing to quell my suspicions. ;-) > > > > > > > > > > > > > > > > > > > > > > Yes, I should delete this #2 from the changelog since it is not very relevant > > > > > > > > > > > (I feel now). My point with #2 was that even if were to raise a softirq > > > > > > > > > > > (which we are not), a scheduler wakeup of ksoftirqd is impossible in this > > > > > > > > > > > path anyway since in_irq() implies in_interrupt(). > > > > > > > > > > > > > > > > > > > > Please! Could you also add a first-principles explanation of why > > > > > > > > > > the added condition is immune from scheduler deadlocks? > > > > > > > > > > > > > > > > > > Sure I can add an example in the change log, however I was thinking of this > > > > > > > > > example which you mentioned: > > > > > > > > > https://lore.kernel.org/lkml/20190627173831.GW26519@xxxxxxxxxxxxx/ > > > > > > > > > > > > > > > > > > previous_reader() > > > > > > > > > { > > > > > > > > > rcu_read_lock(); > > > > > > > > > do_something(); /* Preemption happened here. */ > > > > > > > > > local_irq_disable(); /* Cannot be the scheduler! */ > > > > > > > > > do_something_else(); > > > > > > > > > rcu_read_unlock(); /* Must defer QS, task still queued. */ > > > > > > > > > do_some_other_thing(); > > > > > > > > > local_irq_enable(); > > > > > > > > > } > > > > > > > > > > > > > > > > > > current_reader() /* QS from previous_reader() is still deferred. */ > > > > > > > > > { > > > > > > > > > local_irq_disable(); /* Might be the scheduler. */ > > > > > > > > > do_whatever(); > > > > > > > > > rcu_read_lock(); > > > > > > > > > do_whatever_else(); > > > > > > > > > rcu_read_unlock(); /* Must still defer reporting QS. */ > > > > > > > > > do_whatever_comes_to_mind(); > > > > > > > > > local_irq_enable(); > > > > > > > > > } > > > > > > > > > > > > > > > > > > One modification of the example could be, previous_reader() could also do: > > > > > > > > > previous_reader() > > > > > > > > > { > > > > > > > > > rcu_read_lock(); > > > > > > > > > do_something_that_takes_really_long(); /* causes need_qs in > > > > > > > > > the unlock_special_union to be set */ > > > > > > > > > local_irq_disable(); /* Cannot be the scheduler! */ > > > > > > > > > do_something_else(); > > > > > > > > > rcu_read_unlock(); /* Must defer QS, task still queued. */ > > > > > > > > > do_some_other_thing(); > > > > > > > > > local_irq_enable(); > > > > > > > > > } > > > > > > > > > > > > > > > > The point you were making in that thread being, current_reader() -> > > > > > > > > rcu_read_unlock() -> rcu_read_unlock_special() would not do any wakeups > > > > > > > > because previous_reader() sets the deferred_qs bit. > > > > > > > > > > > > > > > > Anyway, I will add all of this into the changelog. > > > > > > > > > > > > > > Examples are good, but what makes it so that there are no examples of > > > > > > > its being unsafe? > > > > > > > > > > > > > > And a few questions along the way, some quick quiz, some more serious. > > > > > > > Would it be safe if it checked in_interrupt() instead of in_irq()? > > > > > > > If not, should the in_interrupt() in the "if" condition preceding the > > > > > > > added "else if" be changed to in_irq()? Would it make sense to add an > > > > > > > "|| !irqs_were_disabled" do your new "else if" condition? Would the > > > > > > > body of the "else if" actually be executed in current mainline? > > > > > > > > > > > > > > In an attempt to be at least a little constructive, I am doing some > > > > > > > testing of this patch overnight, along with a WARN_ON_ONCE() to see if > > > > > > > that invoke_rcu_core() is ever reached. > > > > > > > > > > > > And that WARN_ON_ONCE() never triggered in two-hour rcutorture runs of > > > > > > TREE01, TREE02, TREE03, and TREE09. (These are the TREE variants in > > > > > > CFLIST that have CONFIG_PREEMPT=y.) > > > > > > > > > > > > This of course raises other questions. But first, do you see that code > > > > > > executing in your testing? > > > > > > > > > > Never mind! Idiot here forgot the "--bootargs rcutree.use_softirq"... > > > > > > > > So this time I ran the test this way: > > > > > > > > tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 8 --duration 10 --configs "TREE01 TREE02 TREE03 TREE09" --bootargs "rcutree.use_softirq=0" > > > > > > > > Still no splats. Though only 10-minute runs instead of the two-hour runs > > > > I did last night. (Got other stuff I need to do, sorry!) > > > > > > > > My test version of your patch is shown below. Please let me know if I messed > > > > something up. > > > > > > I think you also need to pass rcutorture.irqreader=1 ? > > > > > > Otherwise seems all readers happen in process context AFAICS. > > > > Which is the default setting for that, so that's not the issue. > > > > I think one reason could be, in_irq() is false when the timer callback > > executes, since the timer callback is executing after a grace-period. The > > stack is as follows: > > > > Any reason why we cannot both test for call_rcu() and execute the RCU > > callback from the timer hardirq handler? > > > > In fact, I guess on use_nosoftirq systems, the callback will not even run > > in softirq context. > > > > [ 20.553361] => rcu_torture_timer_cb > > [ 20.553361] => rcu_do_batch > > [ 20.553361] => rcu_core > > [ 20.553361] => __do_softirq > > [ 20.553361] => do_softirq_own_stack > > [ 20.553361] => do_softirq.part.16 > > [ 20.553361] => __local_bh_enable_ip > > [ 20.553361] => rcutorture_one_extend > > [ 20.553361] => rcu_torture_one_read > > [ 20.553361] => rcu_torture_reader > > [ 20.553361] => kthread > > [ 20.553361] => ret_from_fork > > Oops! wrong stack trace, this is the one where it shows that the timer handler > is running from softirq, not hardirq. Both the rcu_torture_timer() and > rcu_torture_timer_cb() run from softirq context. ftrace confirms: > > [ 27.949671] rcu_tort-182 8..s1 7268705us : <stack trace> > [ 27.949671] => __ftrace_trace_stack > [ 27.949671] => rcu_torture_timer > [ 27.949671] => call_timer_fn > [ 27.949671] => run_timer_softirq > [ 27.949671] => __do_softirq > [ 27.949671] => irq_exit > [ 27.949671] => smp_apic_timer_interrupt > [ 27.949671] => apic_timer_interrupt > [ 27.949671] => rcutorture_one_extend > [ 27.949671] => rcu_torture_one_read > [ 27.949671] => rcu_torture_reader > [ 27.949671] => kthread > [ 27.949671] => ret_from_fork > > So looks like torture testing modifications are called for, to run them in > hard interrupt context as well to provide this additional coverage.. Or am I > way off in the woods? That actually might be worth doing. The reason I didn't bother is that in the common case, timer softirq generates exactly the same race conditions as would a hard interrupt handler. You can see this in your stack trace, where the call is coming from irq_exit(), that is, from the trailing edge of a hardware interrupt. Thanx, Paul
