rcu: Check for illegal use of RCU from offlined CPUs

Although it is legal to use RCU during early boot, it is anything
but legal to use RCU at runtime from an offlined CPU.  After all, RCU
explicitly ignores offlined CPUs.  This commit therefore adds checks
for runtime use of RCU from offlined CPUs.

These checks are not perfect, in particular, they can be subverted
through use of things like rcu_dereference_raw().  Note that it is not
possible to put checks in rcu_read_lock() and friends due to the fact
that these primitives are used in code that might be used under either
RCU or lock-based protection, which means that checking rcu_read_lock()
gets you fat piles of false positives.

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

1 files changed, 29 insertions, 0 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index dccd2f78db4e..bcf7db2f2fd2 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -591,6 +591,35 @@ int rcu_is_cpu_idle(void)
 }
 EXPORT_SYMBOL(rcu_is_cpu_idle);
 
+#ifdef CONFIG_HOTPLUG_CPU
+
+/*
+ * Is the current CPU online?  Disable preemption to avoid false positives
+ * that could otherwise happen due to the current CPU number being sampled,
+ * this task being preempted, its old CPU being taken offline, resuming
+ * on some other CPU, then determining that its old CPU is now offline.
+ * It is OK to use RCU on an offline processor during initial boot, hence
+ * the check for rcu_scheduler_fully_active.
+ *
+ * Disable checking if in an NMI handler because we cannot safely report
+ * errors from NMI handlers anyway.
+ */
+bool rcu_lockdep_current_cpu_online(void)
+{
+	bool ret;
+
+	if (in_nmi())
+		return 1;
+	preempt_disable();
+	ret = cpu_online(smp_processor_id()) ||
+	      !rcu_scheduler_fully_active;
+	preempt_enable();
+	return ret;
+}
+EXPORT_SYMBOL_GPL(rcu_lockdep_current_cpu_online);
+
+#endif /* #ifdef CONFIG_HOTPLUG_CPU */
+
 #endif /* #ifdef CONFIG_PROVE_RCU */
 
 /**