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    SubjectRe: INFO: possible circular locking dependency detected
    Date
    On Friday 15 July 2011 07:29:22 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
    > > 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
    > > 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?

    Yes.

    I'm off to work - will test any patches when I get back in 8-10 hours.

    Thanks
    Ed

    > 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/#.


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