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

["Paul E. McKenney" <paulmck@linux.ibm.com>]

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