A sporadic hang with consequent crash is observed when booting Hyper-V Gen1
guests:

 Call Trace:
  <IRQ>
  [<ffffffff810ab68d>] ? trace_hardirqs_off+0xd/0x10
  [<ffffffff8107b616>] queue_work_on+0x46/0x90
  [<ffffffff81365696>] ? add_interrupt_randomness+0x176/0x1d0
  ...
  <EOI>
  [<ffffffff81471ddb>] ? _raw_spin_unlock_irqrestore+0x3b/0x60
  [<ffffffff810c295e>] __irq_put_desc_unlock+0x1e/0x40
  [<ffffffff810c5c35>] irq_modify_status+0xb5/0xd0
  [<ffffffff8104adbb>] mp_register_handler+0x4b/0x70
  [<ffffffff8104c55a>] mp_irqdomain_alloc+0x1ea/0x2a0
  [<ffffffff810c7f10>] irq_domain_alloc_irqs_recursive+0x40/0xa0
  [<ffffffff810c860c>] __irq_domain_alloc_irqs+0x13c/0x2b0
  [<ffffffff8104b070>] alloc_isa_irq_from_domain.isra.1+0xc0/0xe0
  [<ffffffff8104bfa5>] mp_map_pin_to_irq+0x165/0x2d0
  [<ffffffff8104c157>] pin_2_irq+0x47/0x80
  [<ffffffff81744253>] setup_IO_APIC+0xfe/0x802
  ...
  [<ffffffff814631c0>] ? rest_init+0x140/0x140

The issue is easily reproducible with a simple instrumentation: if
mdelay(10) is put between mp_setup_entry() and mp_register_handler() calls
in mp_irqdomain_alloc() Hyper-V guest always fails to boot when re-routing
IRQ0. The issue seems to be caused by the fact that we don't disable
interrupts while doing IOPIC programming for legacy IRQs and IRQ0 actually
happens. 

Protect the setup sequence against concurrent interrupts.

[ tglx: Make the protection unconditional and not only for legacy
  	interrupts ]
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Link: http://lkml.kernel.org/r/1444930943-19336-1-git-send-email-vkuznets@redhat.comSigned-off-by: Thomas Gleixner <tglx@linutronix.de>