Re: Badness at kernel/rcutree.c:1228

Hey,

I put the debug function under init/main.c after rcu_init(); but there
is no output on dmesg which means that it receives zero value.

Full dmesg: http://xnet.fi/opt/apps/lkml-2.6.32-vanilla.dmesg.rcu-init.txt

--Mikko Kortelainen

2009/12/8 Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>:
On Tue, Dec 08, 2009 at 11:22:07AM -0800, Paul E. McKenney wrote:
At this point, I must defer to those more skilled than I at diagnosing
early-boot problems.

Well, that is silly on my part -- the problem seems to appear late in
boot, and you had no problem capturing that portion of the boot log.

So please see below for a patch providing a rcu_check_beenonline()
function that, when called after rcu_init(), returns non-zero if the
beenonline fields have become corrupted.  So put calls of the form:

       WARN_ON_ONCE(rcu_check_beenonline());

in the initialization code path preceding the problem.  Either #include
rcupdate.h or explicitly declare the function as appropriate.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---

diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 9642c6b..190a687 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -39,6 +39,8 @@ extern int rcu_cpu_notify(struct notifier_block *self,
 extern int rcu_needs_cpu(int cpu);
 extern int rcu_expedited_torture_stats(char *page);

+extern int rcu_check_beenonline(void);
+
 #ifdef CONFIG_TREE_PREEMPT_RCU

 extern void __rcu_read_lock(void);
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 207125b..27d3722 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -77,6 +77,17 @@ 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);

+/*
+ * Ad-hoc diagnostic function, for use only after rcu_init() has
+ * returned.  Assumes that the boot CPU is CPU 0.  Assumes that
+ * the kernel has been built with CONFIG_TREE_RCU.  Not for inclusion.
+ * Usage: "WARN_ON_ONCE(rcu_check_beenonline());"
+ */
+int rcu_check_beenonline(void)
+{
+       return !per_cpu(rcu_sched_data, 0).beenonline ||
+              !per_cpu(rcu_bh_data, 0).beenonline;
+}

 /*
 * Return true if an RCU grace period is in progress.  The ACCESS_ONCE()s

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