Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs

Joel Fernandes
2019-06-27T14:24:41Z

On Wed, Jun 26, 2019 at 09:25:58AM -0700, Paul E. McKenney wrote:
> On Wed, Jun 26, 2019 at 03:54:47PM +0200, Sebastian Andrzej Siewior wrote:
> > one of my boxes boots with "threadirqs" and since commit 05f415715ce45
> > ("rcu: Speed up expedited GPs when interrupting RCU reader") I run
> > reliably into the following deadlock:
> >
> > | ============================================
> > | WARNING: possible recursive locking detected
> > | 5.2.0-rc6 #279 Not tainted
> > | --------------------------------------------
> > | (cron)/2109 is trying to acquire lock:
> > | 0000000088464daa (&p->pi_lock){-.-.}, at: try_to_wake_up+0x37/0x700
> > |
> > | but task is already holding lock:
> > | 0000000088464daa (&p->pi_lock){-.-.}, at: try_to_wake_up+0x37/0x700
> > |
> > | other info that might help us debug this:
> > | Possible unsafe locking scenario:
> > |
> > | CPU0
> > | ----
> > | lock(&p->pi_lock);
> > | lock(&p->pi_lock);
> > |
> > | *** DEADLOCK ***
> > |
> > | May be due to missing lock nesting notation
> > |
> > | 4 locks held by (cron)/2109:
> > | #0: 00000000c0ae63d9 (&sb->s_type->i_mutex_key){++++}, at: iterate_dir+0x3d/0x170
> > | #1: 0000000088464daa (&p->pi_lock){-.-.}, at: try_to_wake_up+0x37/0x700
> > | #2: 00000000f62f14cf (&rq->lock){-.-.}, at: try_to_wake_up+0x209/0x700
> > | #3: 000000000d32568e (rcu_read_lock){....}, at: cpuacct_charge+0x37/0x1e0
> > |
> > | stack backtrace:
> > | CPU: 3 PID: 2109 Comm: (cron) Not tainted 5.2.0-rc6 #279
> > | Call Trace:
> > | <IRQ>
> > | dump_stack+0x67/0x90
> > | __lock_acquire.cold.63+0x142/0x23a
> > | lock_acquire+0x9b/0x1a0
> > | ? try_to_wake_up+0x37/0x700
> > | _raw_spin_lock_irqsave+0x33/0x50
> > | ? try_to_wake_up+0x37/0x700
> > | try_to_wake_up+0x37/0x700
> > wake up ksoftirqd
> >
> > | rcu_read_unlock_special+0x61/0xa0
> > | __rcu_read_unlock+0x58/0x60
> > | cpuacct_charge+0xeb/0x1e0
> > | update_curr+0x15d/0x350
> > | enqueue_entity+0x115/0x7e0
> > | enqueue_task_fair+0x78/0x450
> > | activate_task+0x41/0x90
> > | ttwu_do_activate+0x49/0x80
> > | try_to_wake_up+0x23f/0x700
> >
> > wake up ksoftirqd
> >
> > | irq_exit+0xba/0xc0
> > | smp_apic_timer_interrupt+0xb2/0x2a0
> > | apic_timer_interrupt+0xf/0x20
> > | </IRQ>
> >
> > based one the commit it seems the problem was always there but now the
> > mix of raise_softirq_irqoff() and set_tsk_need_resched() seems to hit
> > the window quite reliably. Replacing it with
> >
> > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> > index 1102765f91fd1..baab36f4d0f45 100644
> > --- a/kernel/rcu/tree_plugin.h
> > +++ b/kernel/rcu/tree_plugin.h
> > @@ -627,14 +627,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
> > if (preempt_bh_were_disabled || irqs_were_disabled) {
> > WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
> > /* Need to defer quiescent state until everything is enabled. */
> > - if (irqs_were_disabled) {
> > - /* Enabling irqs does not reschedule, so... */
> > - raise_softirq_irqoff(RCU_SOFTIRQ);
> > - } else {
> > - /* Enabling BH or preempt does reschedule, so... */
> > - set_tsk_need_resched(current);
> > - set_preempt_need_resched();
> > - }
> > + raise_softirq_irqoff(RCU_SOFTIRQ);
> > local_irq_restore(flags);
> > return;
> > }
> >
> > will make it go away.
>
> Color me confused. Neither set_tsk_need_resched() nor
> set_preempt_need_resched() acquire locks or do wakeups.
> Yet raise_softirq_irqoff() can do a wakeup if not called
> from hardirq/softirq/NMI context, so I would instead expect
> raise_softirq_irqoff() to be the source of troubles when
> interrupts are threaded.
>
> What am I missing here?

This issue I think is

(in normal process context)
spin_lock_irqsave(rq_lock); // which disables both preemption and interrupt
// but this was done in normal process context,
// not from IRQ handler
rcu_read_lock();
<---------- IPI comes in and sets exp_hint
rcu_read_unlock()
-> rcu_read_unlock_special
-> raise_softirq_irqoff
-> wakeup_softirq <--- because in_interrupt returns false.

I think the issue is in_interrupt() does not know about threaded interrupts.
If it did, then the ksoftirqd wake up would not happen.

Did I get something wrong?

thanks,

- Joel

> > Any suggestions?
>
> Does something like IRQ work help? Please see -rcu commit 0864f057b050
> ("rcu: Use irq_work to get scheduler's attention in clean context")
> for one way of doing this. Perhaps in combination with -rcu commit
> a69987a515c8 ("rcu: Simplify rcu_read_unlock_special() deferred wakeups").
>
> Thanx, Paul