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. > If it had failed, I would have tried the patch shown below. I know that > Sebastian has some concerns about the bug happening anyway, but we have > to start somewhere! ;-) This patch might thus be completely unnecessary. Thanx, Paul > ------------------------------------------------------------------------ > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > index 82c925df1d92..be7bafc2c0a0 100644 > --- a/kernel/rcu/tree_plugin.h > +++ b/kernel/rcu/tree_plugin.h > @@ -624,25 +624,16 @@ static void rcu_read_unlock_special(struct task_struct *t) > (rdp->grpmask & rnp->expmask) || > tick_nohz_full_cpu(rdp->cpu); > // Need to defer quiescent state until everything is enabled. > - if ((exp || in_irq()) && irqs_were_disabled && use_softirq && > - (in_irq() || !t->rcu_read_unlock_special.b.deferred_qs)) { > - // Using softirq, safe to awaken, and we get > - // no help from enabling irqs, unlike bh/preempt. > - raise_softirq_irqoff(RCU_SOFTIRQ); > - } else { > - // Enabling BH or preempt does reschedule, so... > - // Also if no expediting or NO_HZ_FULL, slow is OK. > - set_tsk_need_resched(current); > - set_preempt_need_resched(); > - if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled && > - !rdp->defer_qs_iw_pending && exp) { > - // Get scheduler to re-evaluate and call hooks. > - // If !IRQ_WORK, FQS scan will eventually IPI. > - init_irq_work(&rdp->defer_qs_iw, > - rcu_preempt_deferred_qs_handler); > - rdp->defer_qs_iw_pending = true; > - irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu); > - } > + set_tsk_need_resched(current); > + set_preempt_need_resched(); > + if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled && > + !rdp->defer_qs_iw_pending && exp) { > + // Get scheduler to re-evaluate and call hooks. > + // If !IRQ_WORK, FQS scan will eventually IPI. > + init_irq_work(&rdp->defer_qs_iw, > + rcu_preempt_deferred_qs_handler); > + rdp->defer_qs_iw_pending = true; > + irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu); > } > t->rcu_read_unlock_special.b.deferred_qs = true; > local_irq_restore(flags); >
