Hi Rafael,
A lockdep warning occurs when hot removing a cpu via sysfs:
echo 1 > /sys/bus/acpi/devices/LNXCPU\:02/eject
The kernel is latest upstream, and the test box is a kvm vm,
detail info as following.
[ 221.755113] ======================================================
[ 221.756189] [ INFO: possible circular locking dependency detected ]
[ 221.756189] 3.16.0-rc7+ #118 Not tainted
[ 221.756189] -------------------------------------------------------
[ 221.756189] kworker/u8:3/62 is trying to acquire lock:
[ 221.756189] (s_active#50){++++.+}, at: [<ffffffff811d4ad8>] kernfs_remove_by_name_ns+0x70/0x8c
[ 221.756189]
[ 221.756189] but task is already holding lock:
[ 221.756189] (cpu_hotplug.lock#2){+.+.+.}, at: [<ffffffff810628f9>] cpu_hotplug_begin+0x4f/0x79
[ 221.756189]
[ 221.756189] which lock already depends on the new lock.
[ 221.756189]
[ 221.756189]
[ 221.756189] the existing dependency chain (in reverse order) is:
[ 221.756189]
-> #3 (cpu_hotplug.lock#2){+.+.+.}:
[ 221.756189] [<ffffffff810a1081>] lock_acquire+0xdb/0x101
[ 221.756189] [<ffffffff81596187>] mutex_lock_nested+0x6d/0x37a
[ 221.756189] [<ffffffff810628f9>] cpu_hotplug_begin+0x4f/0x79
[ 221.756189] [<ffffffff81062992>] _cpu_up+0x35/0x126
[ 221.756189] [<ffffffff81062af2>] cpu_up+0x6f/0x81
[ 221.756189] [<ffffffff81d11804>] smp_init+0x4e/0x89
[ 221.756189] [<ffffffff81cf5fe2>] kernel_init_freeable+0x13d/0x24b
[ 221.756189] [<ffffffff8158557a>] kernel_init+0xe/0xdf
[ 221.756189] [<ffffffff8159952c>] ret_from_fork+0x7c/0xb0
[ 221.756189]
-> #2 (cpu_hotplug.lock){++++++}:
[ 221.756189] [<ffffffff810a1081>] lock_acquire+0xdb/0x101
[ 221.756189] [<ffffffff810628eb>] cpu_hotplug_begin+0x41/0x79
[ 221.756189] [<ffffffff81062992>] _cpu_up+0x35/0x126
[ 221.756189] [<ffffffff81062af2>] cpu_up+0x6f/0x81
[ 221.756189] [<ffffffff81d11804>] smp_init+0x4e/0x89
[ 221.756189] [<ffffffff81cf5fe2>] kernel_init_freeable+0x13d/0x24b
[ 221.819219] [<ffffffff8158557a>] kernel_init+0xe/0xdf
[ 221.819219] [<ffffffff8159952c>] ret_from_fork+0x7c/0xb0
[ 221.819219]
[ 221.819219] -> #1 (cpu_add_remove_lock){+.+.+.}:
[ 221.819219] [<ffffffff810a1081>] lock_acquire+0xdb/0x101
[ 221.819219] [<ffffffff815988ab>] _raw_spin_lock_irqsave+0x4d/0x87
[ 221.819219] [<ffffffff81171959>] __delete_object+0x8e/0xab
[ 221.819219] [<ffffffff81171998>] delete_object_full+0x22/0x2e
[ 221.819219] [<ffffffff81587f84>] kmemleak_free+0x56/0x77
[ 221.819219] [<ffffffff81164dee>] slab_free_hook+0x1e/0x5b
[ 221.819219] [<ffffffff81166e21>] kfree+0xac/0x111
[ 221.819219] [<ffffffff812f0768>] acpi_hotplug_work_fn+0x28/0x2d
[ 221.819219] [<ffffffff81078ff5>] process_one_work+0x207/0x375
[ 221.819219] [<ffffffff8107944f>] worker_thread+0x2bd/0x329
[ 221.819219] [<ffffffff8107ef6e>] kthread+0xba/0xc2
[ 221.819219] [<ffffffff8159952c>] ret_from_fork+0x7c/0xb0
[ 221.819219]
[ 221.819219] -> #0 (s_active#50){++++.+}:
[ 221.819219] [<ffffffff810a088a>] __lock_acquire+0xb3b/0xe41
[ 221.819219] [<ffffffff810a1081>] lock_acquire+0xdb/0x101
[ 221.819219] [<ffffffff811d3f97>] __kernfs_remove+0x169/0x2be
[ 221.819219] [<ffffffff811d4ad8>] kernfs_remove_by_name_ns+0x70/0x8c
[ 221.819219] [<ffffffff811d604c>] sysfs_remove_file_ns+0x15/0x17
[ 221.819219] [<ffffffff8138143e>] device_remove_file+0x19/0x1b
[ 221.819219] [<ffffffff813814f6>] device_remove_attrs+0x2e/0x68
[ 221.819219] [<ffffffff81381658>] device_del+0x128/0x187
[ 221.819219] [<ffffffff813816ff>] device_unregister+0x48/0x54
[ 221.819219] [<ffffffff81387366>] unregister_cpu+0x39/0x55
[ 221.819219] [<ffffffff81008c03>] arch_unregister_cpu+0x23/0x28
[ 221.819219] [<ffffffff812f7a75>] acpi_processor_remove+0x91/0xca
[ 221.819219] [<ffffffff812f51d1>] acpi_bus_trim+0x5a/0x8d
[ 221.819219] [<ffffffff812f71c6>] acpi_device_hotplug+0x301/0x3ff
[ 221.819219] [<ffffffff812f0760>] acpi_hotplug_work_fn+0x20/0x2d
[ 221.819219] [<ffffffff81078ff5>] process_one_work+0x207/0x375
[ 221.819219] [<ffffffff8107944f>] worker_thread+0x2bd/0x329
[ 221.819219] [<ffffffff8107ef6e>] kthread+0xba/0xc2
[ 221.819219] [<ffffffff8159952c>] ret_from_fork+0x7c/0xb0
[ 221.819219]
[ 221.819219] other info that might help us debug this:
[ 221.819219]
[ 221.819219] Chain exists of:
[ 221.819219] s_active#50 --> cpu_hotplug.lock --> cpu_hotplug.lock#2
[ 221.819219]
[ 221.819219] Possible unsafe locking scenario:
[ 221.819219]
[ 221.819219] CPU0 CPU1
[ 221.819219] ---- ----
[ 221.819219] lock(cpu_hotplug.lock#2);
[ 221.819219] lock(cpu_hotplug.lock);
[ 221.819219] lock(cpu_hotplug.lock#2);
[ 221.819219] lock(s_active#50);
[ 221.819219]
[ 221.819219] *** DEADLOCK ***
[ 221.819219]
[ 221.819219] 7 locks held by kworker/u8:3/62:
[ 221.819219] #0: ("kacpi_hotplug"){.+.+.+}, at: [<ffffffff81078f4d>] process_one_work+0x15f/0x375
[ 221.819219] #1: ((&hpw->work)){+.+.+.}, at: [<ffffffff81078f4d>] process_one_work+0x15f/0x375
[ 221.819219] #2: (device_hotplug_lock){+.+.+.}, at: [<ffffffff81381f7a>] lock_device_hotplug+0x17/0x19
[ 221.819219] #3: (acpi_scan_lock){+.+.+.}, at: [<ffffffff812f6ef2>] acpi_device_hotplug+0x2d/0x3ff
[ 221.819219] #4: (cpu_add_remove_lock){+.+.+.}, at: [<ffffffff81062722>] cpu_maps_update_begin+0x17/0x19
[ 221.819219] #5: (cpu_hotplug.lock){++++++}, at: [<ffffffff810628af>] cpu_hotplug_begin+0x5/0x79
[ 221.819219] #6: (cpu_hotplug.lock#2){+.+.+.}, at: [<ffffffff810628f9>] cpu_hotplug_begin+0x4f/0x79
[ 221.819219]
[ 221.819219] stack backtrace:
[ 221.819219] CPU: 1 PID: 62 Comm: kworker/u8:3 Not tainted 3.16.0-rc7+ #118
[ 221.819219] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
[ 221.819219] Workqueue: kacpi_hotplug acpi_hotplug_work_fn
[ 221.819219] ffffffff8209cc80 ffff8800372eb968 ffffffff8159266c 00000000000011cf
[ 221.819219] ffffffff820f2840 ffff8800372eb9b8 ffffffff8158e0ce ffff8800372eb9a8
[ 221.819219] ffffffff82485ff0 ffff8800372e2490 ffff8800372e2e68 ffff8800372e2490
[ 221.819219] Call Trace:
[ 221.819219] [<ffffffff8159266c>] dump_stack+0x4e/0x68
[ 221.819219] [<ffffffff8158e0ce>] print_circular_bug+0x1f8/0x209
[ 221.819219] [<ffffffff810a088a>] __lock_acquire+0xb3b/0xe41
[ 221.819219] [<ffffffff811d4ad8>] ? kernfs_remove_by_name_ns+0x70/0x8c
[ 221.819219] [<ffffffff810a1081>] lock_acquire+0xdb/0x101
[ 221.819219] [<ffffffff811d4ad8>] ? kernfs_remove_by_name_ns+0x70/0x8c
[ 221.819219] [<ffffffff811d3f97>] __kernfs_remove+0x169/0x2be
[ 221.819219] [<ffffffff811d4ad8>] ? kernfs_remove_by_name_ns+0x70/0x8c
[ 221.819219] [<ffffffff811d3412>] ? kernfs_find_ns+0xdc/0x104
[ 221.819219] [<ffffffff811d4ad8>] kernfs_remove_by_name_ns+0x70/0x8c
[ 221.819219] [<ffffffff811d604c>] sysfs_remove_file_ns+0x15/0x17
[ 221.819219] [<ffffffff8138143e>] device_remove_file+0x19/0x1b
[ 221.819219] [<ffffffff813814f6>] device_remove_attrs+0x2e/0x68
[ 221.819219] [<ffffffff81381658>] device_del+0x128/0x187
[ 221.819219] [<ffffffff813816ff>] device_unregister+0x48/0x54
[ 221.819219] [<ffffffff81387366>] unregister_cpu+0x39/0x55
[ 221.819219] [<ffffffff81008c03>] arch_unregister_cpu+0x23/0x28
[ 221.819219] [<ffffffff812f7a75>] acpi_processor_remove+0x91/0xca
[ 221.819219] [<ffffffff812f51d1>] acpi_bus_trim+0x5a/0x8d
[ 221.819219] [<ffffffff812f71c6>] acpi_device_hotplug+0x301/0x3ff
[ 221.819219] [<ffffffff812f0760>] acpi_hotplug_work_fn+0x20/0x2d
[ 221.819219] [<ffffffff81078ff5>] process_one_work+0x207/0x375
[ 221.819219] [<ffffffff81078f4d>] ? process_one_work+0x15f/0x375
[ 221.819219] [<ffffffff8107944f>] worker_thread+0x2bd/0x329
[ 221.819219] [<ffffffff81079192>] ? process_scheduled_works+0x2f/0x2f
[ 221.819219] [<ffffffff8107ef6e>] kthread+0xba/0xc2
[ 221.819219] [<ffffffff810a1511>] ? trace_hardirqs_on+0xd/0xf
[ 221.819219] [<ffffffff8107eeb4>] ? __init_kthread_worker+0x59/0x59
[ 221.819219] [<ffffffff8159952c>] ret_from_fork+0x7c/0xb0
[ 221.819219] [<ffffffff8107eeb4>] ? __init_kthread_worker+0x59/0x59
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