linux-kernel - Re: linux-next 20111025: warnings in rcu_idle_exit_common()/rcu_idle_enter

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Date:	Wed, 2 Nov 2011 07:44:03 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Carsten Emde <cbe@...dl.org>
Subject: Re: linux-next 20111025: warnings in
 rcu_idle_exit_common()/rcu_idle_enter_common()

On Wed, Nov 02, 2011 at 12:32:12AM +0800, Wu Fengguang wrote:
> > Hmmmm...  Please see below for a diagnostic patch that prints out who
> > the kernel believes the idle thread is.  Could you please give this
> > a go?
> 
> Sure. Here it goes :)

And here is a possible fix.  Could you please test it?

							Thanx, Paul

------------------------------------------------------------------------

rcu: Fix idle-task checks

RCU has traditionally relied on idle_cpu() to determine whether a given
CPU is running in the context of an idle task, but recent changes have
invalidated this approach.  This commit therefore switches from idle_cpu
to "current->pid != 0".

Suggested-by: Carsten Emde <C.Emde@...dl.org>
Signed-off-by: Paul E. McKenney <paul.mckenney@...aro.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
index f4e7bc3..35f8a07 100644
--- a/kernel/rcutiny.c
+++ b/kernel/rcutiny.c
@@ -65,7 +65,7 @@ static void rcu_idle_enter_common(long long oldval)
 		return;
 	}
 	RCU_TRACE(trace_rcu_dyntick("Start", oldval, rcu_dynticks_nesting));
-	if (!idle_cpu(smp_processor_id())) {
+	if (current->pid != 0) {
 		struct task_struct *idle = idle_task(smp_processor_id());
 
 		RCU_TRACE(trace_rcu_dyntick("Error on entry: not idle task",
@@ -119,7 +119,7 @@ static void rcu_idle_exit_common(long long oldval)
 		return;
 	}
 	RCU_TRACE(trace_rcu_dyntick("End", oldval, rcu_dynticks_nesting));
-	if (!idle_cpu(smp_processor_id())) {
+	if (!current->pid != 0) {
 		struct task_struct *idle = idle_task(smp_processor_id());
 
 		RCU_TRACE(trace_rcu_dyntick("Error on exit: not idle task",
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 3d7b474..414af68 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -355,7 +355,7 @@ static void rcu_idle_enter_common(struct rcu_dynticks *rdtp, long long oldval)
 		return;
 	}
 	trace_rcu_dyntick("Start", oldval, rdtp->dynticks_nesting);
-	if (!idle_cpu(smp_processor_id())) {
+	if (current->pid != 0) {
 		struct task_struct *idle = idle_task(smp_processor_id());
 
 		trace_rcu_dyntick("Error on entry: not idle task",
@@ -449,7 +449,7 @@ static void rcu_idle_exit_common(struct rcu_dynticks *rdtp, long long oldval)
 	smp_mb__after_atomic_inc();  /* See above. */
 	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
 	trace_rcu_dyntick("End", oldval, rdtp->dynticks_nesting);
-	if (!idle_cpu(smp_processor_id())) {
+	if (current->pid != 0) {
 		struct task_struct *idle = idle_task(smp_processor_id());
 
 		trace_rcu_dyntick("Error on exit: not idle task",

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