On Thu, Jun 27, 2019 at 02:51:03PM -0400, Joel Fernandes wrote: > On Thu, Jun 27, 2019 at 02:27:22PM -0400, Joel Fernandes 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" > > > > I built x86_64_defconfig with CONFIG_PREEMPT enabled, then I ran it with > > following boot params: > > rcutorture.shutdown_secs=60 rcutorture.n_barrier_cbs=4 rcutree.kthread_prio=2 > > > > and also "threadirqs" > > > > This was not a TREE config, but just my simple RCU test using qemu. > > Ah, it seems that the issue is reproducible in Linus tree only (which matches > the initial diff Sebastian posted). It cannot be reproduced with your /dev > branch. So perhaps the in_irq() check indeed works. > > Looking further, in_irq() does also set the HARDIRQ_MASK in the preempt_count > courtesy of: > #define __irq_enter() \ > do { \ > account_irq_enter_time(current); \ > preempt_count_add(HARDIRQ_OFFSET); \ > trace_hardirq_enter(); > > I dumped the stack at this point as well even with "threadirqs" just to > double confirm that is the case. > > So probably, the in_irq() check is sufficient. However I am still a bit > nervous about this issue manifesting in other paths of the scheduler > that don't execute from an interrupt handler, but still would have RCU > reader sections with spinlocks held - I am not sure if this is possible > though but it does make me nervous. I have gotten back to this -rcu commit: 385b599e8c04 ("rcu: Allow rcu_read_unlock_special() to raise_softirq() if in_irq()") It works there, and that will be part of my pull request later today. I am continuing an informal manual bisection. ;-) Thanx, Paul > Thanks! > > > > > > > I will try this diff and let you know. > > > > > 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! ;-) > > > > > > ------------------------------------------------------------------------ > > > > > > 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); > > > > Nice to see the code here got simplified ;-) > >
