[RFC PATCH] rcu: don't ignore preempt_disable() in the idle loop

[Lai Jiangshan]

Current, synchronize_sched() ignores preempt-disable()
sequences in the idle loop. It makes synchronize_sched()
is not so pure, and it hurts tracing.

Paul have a proposal before:
http://lkml.org/lkml/2009/4/5/140
http://lkml.org/lkml/2009/4/6/496
But old fix needs to hack into all architectures' idle loops.

This is another try, it uses the fact that idle loops
are executing with preept_count()=1.
But I didn't look deep into all idle loops.

Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
---
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 3ec8160..0761723 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -80,6 +80,10 @@ DEFINE_PER_CPU(struct rcu_data, rcu_sched_data);
 struct rcu_state rcu_bh_state = RCU_STATE_INITIALIZER(rcu_bh_state);
 DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);
 
+#ifndef IDLE_CORE_LOOP_PREEMPT_COUNT
+#define IDLE_CORE_LOOP_PREEMPT_COUNT (1)
+#endif
+
 /*
  * Return true if an RCU grace period is in progress.  The ACCESS_ONCE()s
  * permit this function to be invoked without holding the root rcu_node
@@ -1114,6 +1118,26 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
 		raise_softirq(RCU_SOFTIRQ);
 }
 
+static inline int rcu_idle_qs(int cpu)
+{
+	if (!idle_cpu(cpu))
+		return 0;
+
+	if (!rcu_scheduler_active)
+		return 0;
+
+	if (in_softirq())
+		return 0;
+
+	if (hardirq_count() > (1 << HARDIRQ_SHIFT))
+		return 0;
+
+	if ((preempt_count() & PREEMPT_MASK) > IDLE_CORE_LOOP_PREEMPT_COUNT)
+		return 0;
+
+	return 1;
+}
+
 /*
  * Check to see if this CPU is in a non-context-switch quiescent state
  * (user mode or idle loop for rcu, non-softirq execution for rcu_bh).
@@ -1127,9 +1151,7 @@ void rcu_check_callbacks(int cpu, int user)
 {
 	if (!rcu_pending(cpu))
 		return; /* if nothing for RCU to do. */
-	if (user ||
-	    (idle_cpu(cpu) && rcu_scheduler_active &&
-	     !in_softirq() && hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
+	if (user || rcu_idle_qs(cpu)) {
 
 		/*
 		 * Get here if this CPU took its interrupt from user

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