Suspending a CPU on a RT kernel results in the following backtrace:

| Disabling non-boot CPUs ...
| BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:917
| in_atomic(): 1, irqs_disabled(): 128, pid: 18, name: migration/1
| INFO: lockdep is turned off.
| irq event stamp: 122
| hardirqs last  enabled at (121): [<c06ac0ac>] _raw_spin_unlock_irqrestore+0x88/0x90
| hardirqs last disabled at (122): [<c06abed0>] _raw_spin_lock_irq+0x28/0x5c
|  CPU: 1 PID: 18 Comm: migration/1 Tainted: G        W       4.1.4-rt3-01046-g96ac8da #204
| Hardware name: Generic DRA74X (Flattened Device Tree)
| [<c0019134>] (unwind_backtrace) from [<c0014774>] (show_stack+0x20/0x24)
| [<c0014774>] (show_stack) from [<c06a70f4>] (dump_stack+0x88/0xdc)
| [<c06a70f4>] (dump_stack) from [<c006cab8>] (___might_sleep+0x198/0x2a8)
| [<c006cab8>] (___might_sleep) from [<c06ac4dc>] (rt_spin_lock+0x30/0x70)
| [<c06ac4dc>] (rt_spin_lock) from [<c013f790>] (find_lock_task_mm+0x9c/0x174)
| [<c013f790>] (find_lock_task_mm) from [<c00409ac>] (clear_tasks_mm_cpumask+0xb4/0x1ac)
| [<c00409ac>] (clear_tasks_mm_cpumask) from [<c00166a4>] (__cpu_disable+0x98/0xbc)
| [<c00166a4>] (__cpu_disable) from [<c06a2e8c>] (take_cpu_down+0x1c/0x50)
| [<c06a2e8c>] (take_cpu_down) from [<c00f2600>] (multi_cpu_stop+0x11c/0x158)
| [<c00f2600>] (multi_cpu_stop) from [<c00f2a9c>] (cpu_stopper_thread+0xc4/0x184)
| [<c00f2a9c>] (cpu_stopper_thread) from [<c0069058>] (smpboot_thread_fn+0x18c/0x324)
| [<c0069058>] (smpboot_thread_fn) from [<c00649c4>] (kthread+0xe8/0x104)
| [<c00649c4>] (kthread) from [<c0010058>] (ret_from_fork+0x14/0x3c)
| CPU1: shutdown

The root cause of above backtrace is task_lock() which takes a sleeping
lock on -RT.

To fix the issue, move clear_tasks_mm_cpumask() call from __cpu_disable()
to __cpu_die() which is called on the thread which is asking for a target
CPU to be shutdown. In addition, this change restores CPU hotplug
functionality on ARM CPU1 can be unplugged/plugged many times.

Link: http://lkml.kernel.org/r/1441995683-30817-1-git-send-email-grygorii.strashko@ti.com
[bigeasy: slighty edited the commit message]
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Cc: <linux-arm-kernel@lists.infradead.org>
Cc: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>