On Thu, Jun 27, 2019 at 11:30:22AM -0700, Paul E. McKenney wrote: > On Thu, Jun 27, 2019 at 11:11:12AM -0700, Paul E. McKenney wrote: > > On Thu, Jun 27, 2019 at 01:46:27PM -0400, Joel Fernandes wrote: > > > On Thu, Jun 27, 2019 at 1:43 PM Joel Fernandes <joel@xxxxxxxxxxxxxxxxx> wrote: > > > > > > > > On Thu, Jun 27, 2019 at 11:40 AM Sebastian Andrzej Siewior > > > > <bigeasy@xxxxxxxxxxxxx> wrote: > > > > > > > > > > On 2019-06-27 11:37:10 [-0400], Joel Fernandes wrote: > > > > > > Sebastian it would be nice if possible to trace where the > > > > > > t->rcu_read_unlock_special is set for this scenario of calling > > > > > > rcu_read_unlock_special, to give a clear idea about whether it was > > > > > > really because of an IPI. I guess we could also add additional RCU > > > > > > debug fields to task_struct (just for debugging) to see where there > > > > > > unlock_special is set. > > > > > > > > > > > > Is there a test to reproduce this, or do I just boot an intel x86_64 > > > > > > machine with "threadirqs" and run into it? > > > > > > > > > > Do you want to send me a patch or should I send you my kvm image which > > > > > triggers the bug on boot? > > > > > > > > I could reproduce this as well just booting Linus tree with threadirqs > > > > command line and running rcutorture. In 15 seconds or so it locks > > > > up... gdb backtrace shows the recursive lock: > > > > > > Sorry that got badly wrapped, so I pasted it here: > > > https://hastebin.com/ajivofomik.shell > > > > Which rcutorture scenario would that be? TREE03 is thus far refusing > > to fail for me when run this way: > > > > $ tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 8 --duration 5 --trust-make --configs "TREE03" --bootargs "threadirqs" > > Ah, but I was running -rcu. TREE03 fails at 38 seconds for me on v5.2. > > Now to find out which -rcu commit fixed it. Or at least made it much > less probable, to Sebastian's point. And it still works before this one: a69987a515c8 ("rcu: Simplify rcu_read_unlock_special() deferred wakeups") (This is included in the dev branch of the -rcu tree, and is currently slated for v5.4.) And it works at these commits: 0864f057b050 ("rcu: Use irq_work to get scheduler's attention in clean context") 385b599e8c04 ("rcu: Allow rcu_read_unlock_special() to raise_softirq() if in_irq()") 25102de65fdd ("rcu: Only do rcu_read_unlock_special() wakeups if expedited") 23634ebc1d94 ("rcu: Check for wakeup-safe conditions in rcu_read_unlock_special()") I checked the last one twice, both completing without problems other than false positives due to security-induced pointer obfuscation. (These will be included in my v5.3 pull request.) But not at this commit: 48d07c04b4cc ("rcu: Enable elimination of Tree-RCU softirq processing") This gets RCU CPU stall warnings rather than the double wakeup of ksoftirqd. Works fine without traceirqs. v5.2-rc1 locks up hard at early boot (three of three attempts): [ 2.525153] clocksource: Switched to clocksource tsc [ 2.878858] random: fast init done [ 2.881122] input: ImExPS/2 Generic Explorer Mouse as /devices/platform/i8042/serio1/input/input3 [ 2.884183] probe of serio1 returned 1 after 671008 usecs [ 9.969474] hrtimer: interrupt took 3992554 ns v5.1 gets the double wakeup of ksoftirqd. So it looks like I need to get that pull request sent out, doesn't it? ;-) Thanx, Paul
