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

Paul E. McKenney
2019-06-26T16:27:12Z

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?

> 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