[PATCH] sched: disable preempt in idle tasks

Run idle threads with preempt disabled.

Also corrected a bugs in arm26's cpu_idle (make it actually call schedule()).
How did it ever work before?

Might fix the CPU hotplugging hang which Nigel Cunningham noted.

We think the bug hits if the idle thread is preempted after checking
need_resched() and before going to sleep, then the CPU offlined.

After calling stop_machine_run, the CPU eventually returns from preemption and
into the idle thread and goes to sleep.  The CPU will continue executing
previous idle and have no chance to call play_dead.

By disabling preemption until we are ready to explicitly schedule, this bug is
fixed and the idle threads generally become more robust.

From: alexs <ashepard@u.washington.edu>

  PPC build fix

From: Yoichi Yuasa <yuasa@hh.iij4u.or.jp>

  MIPS build fix

Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Yoichi Yuasa <yuasa@hh.iij4u.or.jp>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 6 insertions, 3 deletions

diff --git a/arch/arm/kernel/process.c b/arch/arm/kernel/process.c
index ba298277becd..93dd92cc12f8 100644
--- a/arch/arm/kernel/process.c
+++ b/arch/arm/kernel/process.c
@@ -116,13 +116,13 @@ void cpu_idle(void)
 
                 if (!idle)
                         idle = default_idle;
-                preempt_disable();
                 leds_event(led_idle_start);
                 while (!need_resched())
                         idle();
                 leds_event(led_idle_end);
-                preempt_enable();
+                preempt_enable_no_resched();
                 schedule();
+                preempt_disable();
         }
 }
 
diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index 77e2e9ca89fa..e55ea952f7aa 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -256,7 +256,9 @@ void __cpuexit cpu_die(void)
 asmlinkage void __cpuinit secondary_start_kernel(void)
 {
         struct mm_struct *mm = &init_mm;
-        unsigned int cpu = smp_processor_id();
+        unsigned int cpu;
+
+        cpu = smp_processor_id();
 
         printk("CPU%u: Booted secondary processor\n", cpu);
 
@@ -273,6 +275,7 @@ asmlinkage void __cpuinit secondary_start_kernel(void)
         local_flush_tlb_all();
 
         cpu_init();
+        preempt_disable();
 
         /*
          * Give the platform a chance to do its own initialisation.