Messages in this thread | | | Date | Fri, 15 Jul 2011 07:36:51 -0700 | From | "Paul E. McKenney" <> | Subject | Re: INFO: possible circular locking dependency detected |
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On Fri, Jul 15, 2011 at 03:07:39PM +0200, Peter Zijlstra wrote: > On Fri, 2011-07-15 at 05:42 -0700, Paul E. McKenney wrote: > > On Fri, Jul 15, 2011 at 01:29:22PM +0200, Peter Zijlstra wrote: > > > On Fri, 2011-07-15 at 07:05 -0400, Ed Tomlinson wrote: > > > > Jul 14 23:21:18 grover kernel: [ 920.659426] -> #1 (rcu_node_level_0){..-...}: > > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [<ffffffff8108b7e5>] lock_acquire+0x95/0x140 > > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [<ffffffff8157808b>] _raw_spin_lock+0x3b/0x50 > > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [<ffffffff810ba797>] __rcu_read_unlock+0x197/0x2d0
Yow... Looks like rcu_read_unlock_special() is being inlined into __rcu_read_unlock().
> > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [<ffffffff8103f2f5>] select_task_rq_fair+0x585/0xa80 > > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [<ffffffff8104633b>] try_to_wake_up+0x17b/0x360 > > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [<ffffffff81046575>] wake_up_process+0x15/0x20 > > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [<ffffffff810528f4>] irq_exit+0xb4/0x100
OK, so all the above stuff is in the context of an irq handler, right?
In which case, why didn't the in_irq() check kick us out before we had a chance to attempt to acquire any locks?
> > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [<ffffffff8158197e>] smp_apic_timer_interrupt+0x6e/0x99 > > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [<ffffffff81580c53>] apic_timer_interrupt+0x13/0x20 > > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [<ffffffff810ba6e9>] __rcu_read_unlock+0xe9/0x2d0 > > > > Jul 14 23:21:18 grover kernel: [ 920.659426] [<ffffffff814c20d4>] sock_def_readable+0x94/0xc0 > > > > > > Ed, are you perchance running with force_irqthreads?
Ah! Would that mean that local_irq_save() gets mapped to locking? Now -that- could be exciting! ;-)
> > > Paul, what appears to be happening here is that some rcu_read_unlock() > > > gets interrupted, possibly before calling rcu_read_unlock_special(), > > > possibly not if the interrupt is itself the timer interrupt. > > > > > > Supposing ->rcu_read_unlock_special is set before, any wakeup happening > > > from an interrupt hitting __rcu_read_unlock(): > > > > > > void __rcu_read_unlock(void) > > > { > > > struct task_struct *t = current; > > > > > > barrier(); /* needed if we ever invoke rcu_read_unlock in rcutree.c */ > > > --t->rcu_read_lock_nesting; > > > barrier(); /* decrement before load of ->rcu_read_unlock_special */ > > > if (t->rcu_read_lock_nesting == 0 && > > > unlikely(ACCESS_ONCE(t->rcu_read_unlock_special))) > > > rcu_read_unlock_special(t); > > > #ifdef CONFIG_PROVE_LOCKING > > > WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0); > > > #endif /* #ifdef CONFIG_PROVE_LOCKING */ > > > } > > > > > > After --t->rcu_read_lock_nesting, but before calling > > > rcu_read_unlock_special(), will trigger this lock inversion. > > > > > > The alternative case, ->rcu_read_unlock_special is not set yet, it can > > > be set if the interrupt hitting in that same spot above, is the timer > > > interrupt, and the wakeup happens either from the softirq ran from the > > > hard IRQ tail, or as I suspect here happens, the wakeup of ksoftirqd/#. > > OK, so the latter case cannot happen (rcu_preempt_check_callbacks only > sets NEED_QS when rcu_read_lock_nesting), we need two interrupts for > this to happen. > > rcu_read_lock() > > <IRQ> > |= RCU_READ_UNLOCK_NEED_QS > > rcu_read_unlock() > __rcu_read_unlock() > --rcu_read_lock_nesting; > <IRQ> > ttwu() > rcu_read_lock() > rcu_read_unlock() > rcu_read_unlock_special() > *BANG* > rcu_read_unlock_special()
The "*BANG*" indicating that the upper-level rcu_read_unlock_special() might overwrite the lower-level rcu_read_unlock_special()'s attempt to clear RCU_READ_UNLOCK_NEED_QS? (Which I believe, perhaps incorrectly, to be prevented by the fact that all modifications to ->rcu_read_unlock_special are carried out with irqs disabled on the corresponding CPU, at least given no RCU_BOOST.) The check for in_irq() should prevent the from-irq rcu_read_unlock_special() from attempting to acquire any locks.
Or am I missing the point of your example?
On the other points, to the extent that I have analyzed them so far:
1. If the task is preempted after the --rcu_read_lock_nesting, it won't see it as being in an RCU read-side critical section, so won't queue it.
2. Of course, the task might have preempted earlier. In this case, the RCU_READ_UNLOCK_BLOCKED is already set, so we will be invoking rcu_read_unlock_special() anyway.
If an interrupt happens between the decrement and the call to rcu_read_unlock_special(), then, yes, the irq handler will also call rcu_read_unlock_special() if it calls rcu_read_unlock(), but the check for in_irq() will prevent the irq handler's invocation of rcu_read_unlock_special() from acquiring any locks.
3. It is possible that the task is preempted after the --rcu_read_lock_nesting, in which case the task won't be queued. Of course the task might already be queued if there was an earlier preemption during this same RCU read-side critical section, in which case #2 applies.
In other words, a preemption in __rcu_read_unlock() after the --rcu_read_lock_nesting has no effect on RCU state: either the task was already marked RCU_READ_UNLOCK_BLOCKED, or it wasn't. Either way, rcu_note_context_switch() does not see this task as being in an RCU read-side critical section.
So what am I missing here?
Thanx, Paul
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