No, I have no idea how a path invoked by fsck and a path invoked by acpi
managed to cross paths. Lot of scheduler and acpi function names in the
tracebacks, so, tossing it to everybody plausible in MAINTAINERS.

Saw this during boot this morning:

[   87.294863] =======================================================
[   87.295177] [ INFO: possible circular locking dependency detected ]
[   87.295402] 2.6.39-rc5-mmotm0429 #1
[   87.295531] -------------------------------------------------------
[   87.295727] fsck/3150 is trying to acquire lock:
[   87.295727]  (rcu_node_level_0){..-...}, at: [<ffffffff810904a8>] rcu_read_unlock_special+0x8c/0x1d5
[   87.295727] 
[   87.295727] but task is already holding lock:
[   87.295727]  (&rq->lock){-.-.-.}, at: [<ffffffff81031dfb>] scheduler_ipi+0x34/0x5d
[   87.295727] 
[   87.295727] which lock already depends on the new lock.
[   87.295727] 
[   87.295727] 
[   87.295727] the existing dependency chain (in reverse order) is:
[   87.295727] 
[   87.295727] -> #2 (&rq->lock){-.-.-.}:
[   87.295727]        [<ffffffff810670e1>] check_prevs_add+0x8b/0x104
[   87.295727]        [<ffffffff810674c9>] validate_chain+0x36f/0x3ab
[   87.295727]        [<ffffffff81067b93>] __lock_acquire+0x369/0x3e2
[   87.295727]        [<ffffffff81068137>] lock_acquire+0xfc/0x14c
[   87.295727]        [<ffffffff81567951>] _raw_spin_lock+0x36/0x45
[   87.295727]        [<ffffffff81027df9>] __task_rq_lock+0x8b/0xd3
[   87.295727]        [<ffffffff81032fd1>] wake_up_new_task+0x41/0x108
[   87.295727]        [<ffffffff81037714>] do_fork+0x265/0x33f
[   87.295727]        [<ffffffff81007d02>] kernel_thread+0x6b/0x6d
[   87.295727]        [<ffffffff81538bbd>] rest_init+0x21/0xd2
[   87.295727]        [<ffffffff81b20b4f>] start_kernel+0x3bb/0x3c6
[   87.295727]        [<ffffffff81b2029f>] x86_64_start_reservations+0xaf/0xb3
[   87.295727]        [<ffffffff81b20393>] x86_64_start_kernel+0xf0/0xf7
[   87.295727] 
[   87.295727] -> #1 (&p->pi_lock){-.-.-.}:
[   87.295727]        [<ffffffff810670e1>] check_prevs_add+0x8b/0x104
[   87.295727]        [<ffffffff810674c9>] validate_chain+0x36f/0x3ab
[   87.295727]        [<ffffffff81067b93>] __lock_acquire+0x369/0x3e2
[   87.295727]        [<ffffffff81068137>] lock_acquire+0xfc/0x14c
[   87.295727]        [<ffffffff81567a4a>] _raw_spin_lock_irqsave+0x44/0x57
[   87.295727]        [<ffffffff81032de1>] try_to_wake_up+0x29/0x1aa
[   87.295727]        [<ffffffff81032f8e>] wake_up_process+0x10/0x12
[   87.295727]        [<ffffffff8108ff01>] rcu_cpu_kthread_timer+0x44/0x58
[   87.295727]        [<ffffffff810452ce>] call_timer_fn+0xac/0x1e9
[   87.295727]        [<ffffffff810455b5>] run_timer_softirq+0x1aa/0x1f2
[   87.295727]        [<ffffffff8103e4cf>] __do_softirq+0x109/0x26a
[   87.295727]        [<ffffffff8156f5cc>] call_softirq+0x1c/0x30
[   87.295727]        [<ffffffff81003207>] do_softirq+0x44/0xf1
[   87.295727]        [<ffffffff8103e901>] irq_exit+0x58/0xc8
[   87.295727]        [<ffffffff81017f5a>] smp_apic_timer_interrupt+0x79/0x87
[   87.295727]        [<ffffffff8156f153>] apic_timer_interrupt+0x13/0x20
[   87.295727]        [<ffffffff81059935>] up+0x55/0x5d
[   87.295727]        [<ffffffff81251dcd>] acpi_os_signal_semaphore+0x1c/0x27
[   87.295727]        [<ffffffff8127459c>] acpi_ut_release_mutex+0x59/0x5d
[   87.295727]        [<ffffffff8126bbb9>] acpi_ns_walk_namespace+0x94/0x17b
[   87.295727]        [<ffffffff8125f202>] acpi_ev_install_space_handler+0x20f/0x225
[   87.295727]        [<ffffffff8125f256>] acpi_ev_install_region_handlers+0x3e/0x77
[   87.295727]        [<ffffffff81272d50>] acpi_enable_subsystem+0x64/0x8b
[   87.295727]        [<ffffffff81b41721>] acpi_bus_init+0x18/0x12a
[   87.295727]        [<ffffffff81b418a1>] acpi_init+0x6e/0xb4
[   87.295727]        [<ffffffff8100020a>] do_one_initcall+0x7a/0x130
[   87.295727]        [<ffffffff81b20c3b>] kernel_init+0xe1/0x15b
[   87.295727]        [<ffffffff8156f4d4>] kernel_thread_helper+0x4/0x10
[   87.295727] 
[   87.295727] -> #0 (rcu_node_level_0){..-...}:
[   87.295727]        [<ffffffff81066eb0>] check_prev_add+0x68/0x20e
[   87.295727]        [<ffffffff810670e1>] check_prevs_add+0x8b/0x104
[   87.295727]        [<ffffffff810674c9>] validate_chain+0x36f/0x3ab
[   87.295727]        [<ffffffff81067b93>] __lock_acquire+0x369/0x3e2
[   87.295727]        [<ffffffff81068137>] lock_acquire+0xfc/0x14c
[   87.295727]        [<ffffffff81567951>] _raw_spin_lock+0x36/0x45
[   87.295727]        [<ffffffff810904a8>] rcu_read_unlock_special+0x8c/0x1d5
[   87.295727]        [<ffffffff81090640>] __rcu_read_unlock+0x4f/0xd7
[   87.295727]        [<ffffffff81027bb3>] rcu_read_unlock+0x21/0x23
[   87.295727]        [<ffffffff8102cc14>] cpuacct_charge+0x6c/0x75
[   87.295727]        [<ffffffff81030c3f>] update_curr+0x101/0x12e
[   87.295727]        [<ffffffff81031149>] check_preempt_wakeup+0xf7/0x23b
[   87.295727]        [<ffffffff8102ac93>] check_preempt_curr+0x2b/0x68
[   87.295727]        [<ffffffff81031cb9>] ttwu_do_wakeup+0x76/0x128
[   87.295727]        [<ffffffff81031dc2>] ttwu_do_activate.constprop.63+0x57/0x5c
[   87.295727]        [<ffffffff81031e0f>] scheduler_ipi+0x48/0x5d
[   87.295727]        [<ffffffff810177d5>] smp_reschedule_interrupt+0x16/0x18
[   87.295727]        [<ffffffff8156f273>] reschedule_interrupt+0x13/0x20
[   87.295727]        [<ffffffff810b6411>] rcu_read_unlock+0x21/0x23
[   87.295727]        [<ffffffff810b70dc>] find_get_page+0xa9/0xb9
[   87.295727]        [<ffffffff810b7840>] do_generic_file_read.constprop.13+0xae/0x46a
[   87.295727]        [<ffffffff810b8640>] generic_file_aio_read+0x1cd/0x232
[   87.295727]        [<ffffffff810f595d>] do_sync_read+0xba/0xfa
[   87.295727]        [<ffffffff810f60d7>] vfs_read+0xde/0x129
[   87.295727]        [<ffffffff810f6167>] sys_read+0x45/0x69
[   87.295727]        [<ffffffff8156e73b>] system_call_fastpath+0x16/0x1b
[   87.295727] 
[   87.295727] other info that might help us debug this:
[   87.295727] 
[   87.295727] Chain exists of:
[   87.295727]   rcu_node_level_0 --> &p->pi_lock --> &rq->lock
[   87.295727] 
[   87.295727]  Possible unsafe locking scenario:
[   87.295727] 
[   87.295727]        CPU0                    CPU1
[   87.295727]        ----                    ----
[   87.295727]   lock(&rq->lock);
[   87.295727]                                lock(&p->pi_lock);
[   87.295727]                                lock(&rq->lock);
[   87.295727]   lock(rcu_node_level_0);
[   87.295727] 
[   87.295727]  *** DEADLOCK ***
[   87.295727] 
[   87.295727] 1 lock held by fsck/3150:
[   87.295727]  #0:  (&rq->lock){-.-.-.}, at: [<ffffffff81031dfb>] scheduler_ipi+0x34/0x5d
[   87.295727] 
[   87.295727] stack backtrace:
[   87.295727] Pid: 3150, comm: fsck Not tainted 2.6.39-rc5-mmotm0429 #1
[   87.295727] Call Trace:
[   87.295727]  <IRQ>  [<ffffffff815486ed>] print_circular_bug+0xc8/0xd9
[   87.295727]  [<ffffffff81066eb0>] check_prev_add+0x68/0x20e
[   87.295727]  [<ffffffff812d863a>] ? scsi_request_fn+0x30d/0x3de
[   87.295727]  [<ffffffff810670e1>] check_prevs_add+0x8b/0x104
[   87.295727]  [<ffffffff810674c9>] validate_chain+0x36f/0x3ab
[   87.295727]  [<ffffffff81067b93>] __lock_acquire+0x369/0x3e2
[   87.295727]  [<ffffffff810685f5>] ? trace_hardirqs_on_caller+0xfd/0x13b
[   87.295727]  [<ffffffff81567fb0>] ? _raw_spin_unlock_irqrestore+0x7b/0x80
[   87.295727]  [<ffffffff810904a8>] ? rcu_read_unlock_special+0x8c/0x1d5
[   87.295727]  [<ffffffff81068137>] lock_acquire+0xfc/0x14c
[   87.295727]  [<ffffffff810904a8>] ? rcu_read_unlock_special+0x8c/0x1d5
[   87.295727]  [<ffffffff81567951>] _raw_spin_lock+0x36/0x45
[   87.295727]  [<ffffffff810904a8>] ? rcu_read_unlock_special+0x8c/0x1d5
[   87.295727]  [<ffffffff810904a8>] rcu_read_unlock_special+0x8c/0x1d5
[   87.295727]  [<ffffffff81067f37>] ? __lock_release+0x8c/0x9c
[   87.295727]  [<ffffffff81090640>] __rcu_read_unlock+0x4f/0xd7
[   87.295727]  [<ffffffff81027bb3>] rcu_read_unlock+0x21/0x23
[   87.295727]  [<ffffffff8102cc14>] cpuacct_charge+0x6c/0x75
[   87.295727]  [<ffffffff81030c3f>] update_curr+0x101/0x12e
[   87.295727]  [<ffffffff81031149>] check_preempt_wakeup+0xf7/0x23b
[   87.295727]  [<ffffffff8102ac93>] check_preempt_curr+0x2b/0x68
[   87.295727]  [<ffffffff81031cb9>] ttwu_do_wakeup+0x76/0x128
[   87.295727]  [<ffffffff81031dc2>] ttwu_do_activate.constprop.63+0x57/0x5c
[   87.295727]  [<ffffffff81031e0f>] scheduler_ipi+0x48/0x5d
[   87.295727]  [<ffffffff810177d5>] smp_reschedule_interrupt+0x16/0x18
[   87.295727]  [<ffffffff8156f273>] reschedule_interrupt+0x13/0x20
[   87.295727]  <EOI>  [<ffffffff8109041d>] ? rcu_read_unlock_special+0x1/0x1d5
[   87.295727]  [<ffffffff81090640>] ? __rcu_read_unlock+0x4f/0xd7
[   87.295727]  [<ffffffff810b6411>] rcu_read_unlock+0x21/0x23
[   87.295727]  [<ffffffff810b70dc>] find_get_page+0xa9/0xb9
[   87.295727]  [<ffffffff810b7840>] do_generic_file_read.constprop.13+0xae/0x46a
[   87.295727]  [<ffffffff810b8640>] generic_file_aio_read+0x1cd/0x232
[   87.295727]  [<ffffffff810f595d>] do_sync_read+0xba/0xfa
[   87.295727]  [<ffffffff810f5cc8>] ? rw_verify_area+0x13e/0x161
[   87.295727]  [<ffffffff810f60d7>] vfs_read+0xde/0x129
[   87.295727]  [<ffffffff810f6167>] sys_read+0x45/0x69
[   87.295727]  [<ffffffff8156e73b>] system_call_fastpath+0x16/0x1b