[PATCH 3/4] tracing: Add stack_tracer_disable/enable() functions

[Steven Rostedt <rostedt@goodmis.org>]

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From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>

There are certain parts of the kernel that can not let stack tracing
proceed (namely in RCU), because the stack tracer uses RCU, and parts of RCU
internals can not handle having RCU read side locks taken.

Add stack_tracer_disable() and stack_tracer_enable() functions to let RCU
stop stack tracing on the current CPU as it is in those critical sections.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
---
 include/linux/ftrace.h     |  6 ++++++
 kernel/trace/trace_stack.c | 28 ++++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index ef7123219f14..40afee35565a 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -286,6 +286,12 @@ int
 stack_trace_sysctl(struct ctl_table *table, int write,
 		   void __user *buffer, size_t *lenp,
 		   loff_t *ppos);
+
+void stack_tracer_disable(void);
+void stack_tracer_enable(void);
+#else
+static inline void stack_tracer_disable(void) { }
+static inline void stack_tracer_enabe(void) { }
 #endif
 
 struct ftrace_func_command {
diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
index 05ad2b86461e..5adbb73ec2ec 100644
--- a/kernel/trace/trace_stack.c
+++ b/kernel/trace/trace_stack.c
@@ -41,6 +41,34 @@ static DEFINE_MUTEX(stack_sysctl_mutex);
 int stack_tracer_enabled;
 static int last_stack_tracer_enabled;
 
+/**
+ * stack_tracer_disable - temporarily disable the stack tracer
+ *
+ * There's a few locations (namely in RCU) where stack tracing
+ * can not be executed. This function is used to disable stack
+ * tracing during those critical sections.
+ *
+ * This function will disable preemption. stack_tracer_enable()
+ * must be called shortly after this is called.
+ */
+void stack_tracer_disable(void)
+{
+	preempt_disable_notrace();
+	this_cpu_inc(trace_active);
+}
+
+/**
+ * stack_tracer_enable - re-enable the stack tracer
+ *
+ * After stack_tracer_disable() is called, stack_tracer_enable()
+ * must shortly be called afterward.
+ */
+void stack_tracer_enable(void)
+{
+	this_cpu_dec(trace_active);
+	preempt_enable_notrace();
+}
+
 void stack_trace_print(void)
 {
 	long i;
-- 
2.10.2