With the enhanced CPU hotplug lockdep coverage the following lockdep splat
happens:

======================================================
WARNING: possible circular locking dependency detected
4.12.0-rc2+ #84 Tainted: G        W      
------------------------------------------------------
cpuhp/1/15 is trying to acquire lock:
flush_work+0x39/0x2f0

but task is already holding lock:
cpuhp_thread_fun+0x30/0x160

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 (cpuhp_state){+.+.+.}:
       lock_acquire+0xb4/0x200
       cpuhp_kick_ap_work+0x72/0x330
       _cpu_down+0x8b/0x100
       do_cpu_down+0x3e/0x60
       cpu_down+0x10/0x20
       cpu_subsys_offline+0x14/0x20
       device_offline+0x88/0xb0
       online_store+0x4c/0xa0
       dev_attr_store+0x18/0x30
       sysfs_kf_write+0x45/0x60
       kernfs_fop_write+0x156/0x1e0
       __vfs_write+0x37/0x160
       vfs_write+0xca/0x1c0
       SyS_write+0x58/0xc0
       entry_SYSCALL_64_fastpath+0x23/0xc2

-> #1 (cpu_hotplug_lock.rw_sem){++++++}:
       lock_acquire+0xb4/0x200
       cpus_read_lock+0x3d/0xb0
       apply_workqueue_attrs+0x17/0x50
       __alloc_workqueue_key+0x1e1/0x530
       scsi_host_alloc+0x373/0x480 [scsi_mod]
       ata_scsi_add_hosts+0xcb/0x130 [libata]
       ata_host_register+0x11a/0x2c0 [libata]
       ata_host_activate+0xf0/0x150 [libata]
       ahci_host_activate+0x13e/0x170 [libahci]
       ahci_init_one+0xa3a/0xd3f [ahci]
       local_pci_probe+0x45/0xa0
       work_for_cpu_fn+0x14/0x20
       process_one_work+0x1f9/0x690
       worker_thread+0x200/0x3d0
       kthread+0x138/0x170
       ret_from_fork+0x31/0x40

-> #0 ((&wfc.work)){+.+.+.}:
       __lock_acquire+0x11e1/0x13e0
       lock_acquire+0xb4/0x200
       flush_work+0x5c/0x2f0
       work_on_cpu+0xa1/0xd0
       acpi_processor_get_throttling+0x3d/0x50
       acpi_processor_reevaluate_tstate+0x2c/0x50
       acpi_soft_cpu_online+0x69/0xd0
       cpuhp_invoke_callback+0xb4/0x8b0
       cpuhp_up_callbacks+0x36/0xc0
       cpuhp_thread_fun+0x14e/0x160
       smpboot_thread_fn+0x1e8/0x300
       kthread+0x138/0x170
       ret_from_fork+0x31/0x40

other info that might help us debug this:

Chain exists of:
  (&wfc.work) --> cpu_hotplug_lock.rw_sem --> cpuhp_state

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(cpuhp_state);
                               lock(cpu_hotplug_lock.rw_sem);
                               lock(cpuhp_state);
  lock((&wfc.work));

 *** DEADLOCK ***

1 lock held by cpuhp/1/15:
cpuhp_thread_fun+0x30/0x160

stack backtrace:
CPU: 1 PID: 15 Comm: cpuhp/1 Tainted: G        W       4.12.0-rc2+ #84
Hardware name: Supermicro SYS-4048B-TR4FT/X10QBi, BIOS 1.1a 07/29/2015
Call Trace:
 dump_stack+0x85/0xc4
 print_circular_bug+0x209/0x217
 __lock_acquire+0x11e1/0x13e0
 lock_acquire+0xb4/0x200
 ? lock_acquire+0xb4/0x200
 ? flush_work+0x39/0x2f0
 ? acpi_processor_start+0x50/0x50
 flush_work+0x5c/0x2f0
 ? flush_work+0x39/0x2f0
 ? acpi_processor_start+0x50/0x50
 ? mark_held_locks+0x6d/0x90
 ? queue_work_on+0x56/0x90
 ? trace_hardirqs_on_caller+0x154/0x1c0
 ? trace_hardirqs_on+0xd/0x10
 ? acpi_processor_start+0x50/0x50
 work_on_cpu+0xa1/0xd0
 ? find_worker_executing_work+0x50/0x50
 ? acpi_processor_power_exit+0x70/0x70
 acpi_processor_get_throttling+0x3d/0x50
 acpi_processor_reevaluate_tstate+0x2c/0x50
 acpi_soft_cpu_online+0x69/0xd0
 cpuhp_invoke_callback+0xb4/0x8b0
 ? lock_acquire+0xb4/0x200
 ? padata_replace+0x120/0x120
 cpuhp_up_callbacks+0x36/0xc0
 cpuhp_thread_fun+0x14e/0x160
 smpboot_thread_fn+0x1e8/0x300
 kthread+0x138/0x170
 ? sort_range+0x30/0x30
 ? kthread_create_on_node+0x70/0x70
 ret_from_fork+0x31/0x40

The problem is that the work is scheduled on the current CPU from the
hotplug thread associated with that CPU.

It's not required to invoke these functions via the workqueue because the
hotplug thread runs on the target CPU already.

Check whether current is a per cpu thread pinned on the target CPU and
invoke the function directly to avoid the workqueue.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-acpi@vger.kernel.org
Cc: Len Brown <lenb@kernel.org>
Link: http://lkml.kernel.org/r/20170524081549.620489733@linutronix.de