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 ? Yes, but rcutorture.irqreader=1 is the default: rcu-torture:--- Start of test: nreaders=7 nfakewriters=4 stat_interval=15 verbose=1 test_no_idle_hz=1 shuffle_interval=3 stutter=5 irqreader=1 fqs_duration=0 fqs_holdoff=0 fqs_stutter=3 test_boost=1/0 test_boost_interval=7 test_boost_duration=4 shutdown_secs=28800 stall_cpu=0 stall_cpu_holdoff=10 stall_cpu_irqsoff=0 n_barrier_cbs=4 onoff_interval=1000 onoff_holdoff=30 Or from the source-code level: torture_param(int, irqreader, 1, "Allow RCU readers from irq handlers"); > Otherwise seems all readers happen in process context AFAICS. Yes, should irqreader=0, all readers would happen in process context. Thanx, Paul > thanks, > > - Joel > > > > Thanx, Paul > > > > ------------------------------------------------------------------------ > > > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > > index 2defc7fe74c3..abf2fbba2568 100644 > > --- a/kernel/rcu/tree_plugin.h > > +++ b/kernel/rcu/tree_plugin.h > > @@ -621,6 +621,10 @@ static void rcu_read_unlock_special(struct task_struct *t) > > // Using softirq, safe to awaken, and we get > > // no help from enabling irqs, unlike bh/preempt. > > raise_softirq_irqoff(RCU_SOFTIRQ); > > + } else if (exp && in_irq() && !use_softirq && > > + !t->rcu_read_unlock_special.b.deferred_qs) { > > + WARN_ON_ONCE(1); // Live code? > > + invoke_rcu_core(); > > } else { > > // Enabling BH or preempt does reschedule, so... > > // Also if no expediting or NO_HZ_FULL, slow is OK.
