[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, 7 insertions, 0 deletions

diff --git a/arch/sparc64/kernel/process.c b/arch/sparc64/kernel/process.c
index 7d10b0397091..2f89206e008f 100644
--- a/arch/sparc64/kernel/process.c
+++ b/arch/sparc64/kernel/process.c
@@ -74,7 +74,9 @@ void cpu_idle(void)
                 while (!need_resched())
                         barrier();
 
+                preempt_enable_no_resched();
                 schedule();
+                preempt_disable();
                 check_pgt_cache();
         }
 }
@@ -93,7 +95,9 @@ void cpu_idle(void)
                 if (need_resched()) {
                         unidle_me();
                         clear_thread_flag(TIF_POLLING_NRFLAG);
+                        preempt_enable_no_resched();
                         schedule();
+                        preempt_disable();
                         set_thread_flag(TIF_POLLING_NRFLAG);
                         check_pgt_cache();
                 }
diff --git a/arch/sparc64/kernel/smp.c b/arch/sparc64/kernel/smp.c
index 5d90ee9aebf1..8aca4b1dc04e 100644
--- a/arch/sparc64/kernel/smp.c
+++ b/arch/sparc64/kernel/smp.c
@@ -168,6 +168,9 @@ void __init smp_callin(void)
                 rmb();
 
         cpu_set(cpuid, cpu_online_map);
+
+        /* idle thread is expected to have preempt disabled */
+        preempt_disable();
 }
 
 void cpu_panic(void)