[PATCH] Plug kdump shutdown race window

lapic_shutdown() re-enables interrupts which is un-desirable for panic
case, so use local_irq_save() and local_irq_restore() to keep the irqs
disabled for kexec on panic case, and close a possible race window while
kdump shutdown as shown in this stack trace

   -- BUG: spinlock lockup on CPU#1, bash/4396, c52781a0
   [<c01c1870>] _raw_spin_lock+0xb7/0xd2
   [<c029e148>] _spin_lock+0x6/0x8
   [<c011b33f>] scheduler_tick+0xe7/0x328
   [<c0128a7c>] update_process_times+0x51/0x5d
   [<c0114592>] smp_apic_timer_interrupt+0x4f/0x58
   [<c01141ff>] lapic_shutdown+0x76/0x7e
   [<c0104d7c>] apic_timer_interrupt+0x1c/0x30
   [<c01141ff>] lapic_shutdown+0x76/0x7e
   [<c0116659>] machine_crash_shutdown+0x83/0xaa
   [<c013cc36>] crash_kexec+0xc1/0xe3
   [<c029e148>] _spin_lock+0x6/0x8
   [<c013cc22>] crash_kexec+0xad/0xe3
   [<c0215280>] __handle_sysrq+0x84/0xfd
   [<c018d937>] write_sysrq_trigger+0x2c/0x35
   [<c015e47b>] vfs_write+0xa2/0x13b
   [<c015ea73>] sys_write+0x3b/0x64
   [<c0103c69>] syscall_call+0x7/0xb
Signed-off-by: Maneesh Soni <maneesh@in.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/i386/kernel/apic.c

@@ -570,16 +570,18 @@ void __devinit setup_local_APIC(void)
  */
 void lapic_shutdown(void)
 {
+	unsigned long flags;
+
 	if (!cpu_has_apic)
 		return;
 
-	local_irq_disable();
+	local_irq_save(flags);
 	clear_local_APIC();
 
 	if (enabled_via_apicbase)
 		disable_local_APIC();
 
-	local_irq_enable();
+	local_irq_restore(flags);
 }
 
 #ifdef CONFIG_PM