From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date: Sun, 21 Aug 2011 01:29:32 +0000 (-0700)
Subject: rcu: Remove rcu_needs_cpu_flush() to avoid false quiescent states
X-Git-Tag: next-20110914~19^2~9
X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=1a1ce0ecccea6d8434df0e54f2e7fcbe01a94c4c;p=karo-tx-linux.git

rcu: Remove rcu_needs_cpu_flush() to avoid false quiescent states

The purpose of rcu_needs_cpu_flush() was to iterate on pushing the
current grace period in order to help the current CPU enter dyntick-idle
mode.  However, this can result in failures if the CPU starts entering
dyntick-idle mode, but then backs out.  In this case, the call to
rcu_pending() from rcu_needs_cpu_flush() might end up announcing a
non-existing quiescent state.

This commit therefore removes rcu_needs_cpu_flush() in favor of letting
the dyntick-idle machinery at the end of the softirq handler push the
loop along via its call to rcu_pending().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 743a658e3df1..ab37c19f386b 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1528,9 +1528,6 @@ static void rcu_process_callbacks(struct softirq_action *unused)
 				&__get_cpu_var(rcu_sched_data));
 	__rcu_process_callbacks(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
 	rcu_preempt_process_callbacks();
-
-	/* If we are last CPU on way to dyntick-idle mode, accelerate it. */
-	rcu_needs_cpu_flush();
 	trace_rcu_utilization("End RCU core");
 }
 
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index f509f728f9fb..849ce9ec51fe 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -458,7 +458,6 @@ static int rcu_preempt_needs_cpu(int cpu);
 static void __cpuinit rcu_preempt_init_percpu_data(int cpu);
 static void rcu_preempt_send_cbs_to_online(void);
 static void __init __rcu_init_preempt(void);
-static void rcu_needs_cpu_flush(void);
 static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags);
 static void rcu_preempt_boost_start_gp(struct rcu_node *rnp);
 static void invoke_rcu_callbacks_kthread(void);
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 509d5a31c8ac..e7eea7453b4f 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1949,15 +1949,6 @@ int rcu_needs_cpu(int cpu)
 	return rcu_needs_cpu_quick_check(cpu);
 }
 
-/*
- * Check to see if we need to continue a callback-flush operations to
- * allow the last CPU to enter dyntick-idle mode.  But fast dyntick-idle
- * entry is not configured, so we never do need to.
- */
-static void rcu_needs_cpu_flush(void)
-{
-}
-
 #else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */
 
 #define RCU_NEEDS_CPU_FLUSHES 5
@@ -2033,20 +2024,4 @@ int rcu_needs_cpu(int cpu)
 	return c;
 }
 
-/*
- * Check to see if we need to continue a callback-flush operations to
- * allow the last CPU to enter dyntick-idle mode.
- */
-static void rcu_needs_cpu_flush(void)
-{
-	int cpu = smp_processor_id();
-	unsigned long flags;
-
-	if (per_cpu(rcu_dyntick_drain, cpu) <= 0)
-		return;
-	local_irq_save(flags);
-	(void)rcu_needs_cpu(cpu);
-	local_irq_restore(flags);
-}
-
 #endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */