[RFC PATCH v2]nohz: protect smp_processor_id() in tick_nohz_task_switch()

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

I saw following error when testing the latest nohz code on Power:

[   85.295384] BUG: using smp_processor_id() in preemptible [00000000] code: rsyslogd/3493
[   85.295396] caller is .tick_nohz_task_switch+0x1c/0xb8
[   85.295402] Call Trace:
[   85.295408] [c0000001fababab0] [c000000000012dc4] .show_stack+0x110/0x25c (unreliable)
[   85.295420] [c0000001fababba0] [c0000000007c4b54] .dump_stack+0x20/0x30
[   85.295430] [c0000001fababc10] [c00000000044eb74] .debug_smp_processor_id+0xf4/0x124
[   85.295438] [c0000001fababca0] [c0000000000d7594] .tick_nohz_task_switch+0x1c/0xb8
[   85.295447] [c0000001fababd20] [c0000000000b9748] .finish_task_switch+0x13c/0x160
[   85.295455] [c0000001fababdb0] [c0000000000bbe50] .schedule_tail+0x50/0x124
[   85.295463] [c0000001fababe30] [c000000000009dc8] .ret_from_fork+0x4/0x54

The code below moves the test into local_irq_save/restore section to
avoid the above complaint.

Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
---
 kernel/time/tick-sched.c |    7 ++++---
 1 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index da53c8f..1c9f53b 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -251,14 +251,15 @@ void tick_nohz_task_switch(struct task_struct *tsk)
 {
 	unsigned long flags;
 
-	if (!tick_nohz_full_cpu(smp_processor_id()))
-		return;
-
 	local_irq_save(flags);
 
+	if (!tick_nohz_full_cpu(smp_processor_id()))
+		goto out;
+
 	if (tick_nohz_tick_stopped() && !can_stop_full_tick())
 		tick_nohz_full_kick();
 
+out:
 	local_irq_restore(flags);
 }
 
-- 
1.7.1