[RFC PATCH powerpc] Fix a lazy irq related WARING in arch_local_irq_restore()

[Li Zhong <zhong@linux.vnet.ibm.com>]

This patch tries to fix a WARNING in irq.c(below), when doing cpu offline/online. 

The reason is that if the preferred offline state is CPU_STATE_INACTIVE, when
cpu offline, pseries_mach_cpu_die() calls extended_cede_processor() to cede
the processor. After the hv call returns, the MSR_EE is enabled, while
the irq_happened in paca should already be set as PACA_IRQ_HARD_DIS.

Then when the cpu is put online again, the warning is reported when
start_secondary() tries to enable irq. [ Sometimes, we don't see this warning,
that's because when we come to local_irq_enable(), there might already have been
some interrupts occurred, e.g. PACA_IRQ_DEC is already set. ]

The patch tries to clear MSR_EE by calling __hard_irq_disable() after cede
returns, and before calling start_secondary_resume(). 
 
[   56.618846] WARNING: at arch/powerpc/kernel/irq.c:240
[   56.618851] Modules linked in: rcutorture ipv6 dm_mod ext3 jbd mbcache sg sd_mod crc_t10dif ibmvscsic scsi_transport_srp scsi_tgt ibmveth
[   56.618883] NIP: c00000000000ff94 LR: c00000000067a5e0 CTR: 0000000000000001
[   56.618889] REGS: c0000001fef6bbe0 TRAP: 0700   Not tainted  (3.6.0-rc1-autokern1)
[   56.618894] MSR: 8000000000029032 <SF,EE,ME,IR,DR,RI>  CR: 42000082  XER: 20000000
[   56.618913] SOFTE: 1
[   56.618916] CFAR: c00000000067a5dc
[   56.618920] TASK = c0000001feed79a0[0] 'swapper/5' THREAD: c0000001fef68000 CPU: 5
GPR00: 0000000000000001 c0000001fef6be60 c000000000f9ca08 0000000000000001 
GPR04: 0000000000000001 0000000000000008 0000000000000001 0000000000000000 
GPR08: 0000000000000000 c0000001feed79a0 0008a80000000000 0000000000000000 
GPR12: 0000000022000082 c00000000f330f00 c0000001fef6bf90 000000000f394b4c 
GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 
GPR20: 0000000000000000 0000000000000000 0000000000000000 0000000000000000 
GPR24: c000000000fe8f80 0000000000000008 0000000000000028 0000000000000000 
GPR28: 0000000000000000 0000000000000020 c000000000f1ab40 0000000000000001 
[   56.619014] NIP [c00000000000ff94] .arch_local_irq_restore+0x34/0xa0
[   56.619020] LR [c00000000067a5e0] .start_secondary+0x368/0x37c
[   56.619025] Call Trace:
[   56.619030] [c0000001fef6be60] [c000000001ba0500] 0xc000000001ba0500 (unreliable)
[   56.619038] [c0000001fef6bed0] [c00000000067a5e0] .start_secondary+0x368/0x37c
[   56.619046] [c0000001fef6bf90] [c000000000009380] .start_secondary_resume+0x10/0x14
[   56.619052] Instruction dump:
[   56.619056] f8010010 f821ff91 986d022a 2fa30000 419e0054 880d022b 78000621 41820048 
[   56.619071] 2f800001 40de0064 7c0000a6 78008fe2 <0b000000> 2fa00000 40de0050 38000000 
[   56.619088] ---[ end trace 0199c0d783d7f9ba ]---

Reported-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
---
 arch/powerpc/platforms/pseries/hotplug-cpu.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
index 64c97d8..8de539a 100644
--- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
+++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
@@ -137,6 +137,8 @@ static void pseries_mach_cpu_die(void)
 		if (get_preferred_offline_state(cpu) == CPU_STATE_ONLINE) {
 			unregister_slb_shadow(hwcpu);
 
+			__hard_irq_disable();
+
 			/*
 			 * Call to start_secondary_resume() will not return.
 			 * Kernel stack will be reset and start_secondary()
-- 
1.7.1