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[for-next][PATCH 3/7] tracing: Add stack_tracer_disable/enable() functions

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

There are certain parts of the kernel that cannot let stack tracing
proceed (namely in RCU), because the stack tracer uses RCU, and parts of RCU
internals cannot 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 when 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 | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index ef7123219f14..7b4e6572ab21 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_enable(void) { }
 #endif
 
 struct ftrace_func_command {
diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
index 338d076a06da..21e536cf66e4 100644
--- a/kernel/trace/trace_stack.c
+++ b/kernel/trace/trace_stack.c
@@ -41,6 +41,38 @@ 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
+ * cannot be executed. This function is used to disable stack
+ * tracing during those critical sections.
+ *
+ * This function must be called with preemption or interrupts
+ * disabled and stack_tracer_enable() must be called shortly after
+ * while preemption or interrupts are still disabled.
+ */
+void stack_tracer_disable(void)
+{
+	/* Preemption or interupts must be disabled */
+	if (IS_ENABLED(CONFIG_PREEMPT_DEBUG))
+		WARN_ON_ONCE(!preempt_count() || !irqs_disabled());
+	this_cpu_inc(trace_active);
+}
+
+/**
+ * stack_tracer_enable - re-enable the stack tracer
+ *
+ * After stack_tracer_disable() is called, stack_tracer_enable()
+ * must be called shortly afterward.
+ */
+void stack_tracer_enable(void)
+{
+	if (IS_ENABLED(CONFIG_PREEMPT_DEBUG))
+		WARN_ON_ONCE(!preempt_count() || !irqs_disabled());
+	this_cpu_dec(trace_active);
+}
+
 void stack_trace_print(void)
 {
 	long i;
-- 
2.10.2

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[for-next][PATCH 0/7] tracing: Stack tracing and rcu modification updates Steven Rostedt <rostedt@goodmis.org> - 2017-04-10 22:00 +0200
  [for-next][PATCH 6/7] rcu/tracing: Add rcu_disabled to denote when rcu_irq_enter() will not  work Steven Rostedt <rostedt@goodmis.org> - 2017-04-10 22:00 +0200
  [for-next][PATCH 4/7] tracing: Rename trace_active to disable_stack_tracer and inline its  modification Steven Rostedt <rostedt@goodmis.org> - 2017-04-10 22:00 +0200
    Re: [for-next][PATCH 4/7] tracing: Rename trace_active to  disable_stack_tracer and inline its modification Valentin Rothberg <valentinrothberg@gmail.com> - 2017-04-13 10:00 +0200
      Re: [for-next][PATCH 4/7] tracing: Rename trace_active to  disable_stack_tracer and inline its modification Steven Rostedt <rostedt@goodmis.org> - 2017-04-13 16:10 +0200
  [for-next][PATCH 7/7] tracing: Make sure rcu_irq_enter() can work for trace_*_rcuidle()  trace events Steven Rostedt <rostedt@goodmis.org> - 2017-04-10 22:00 +0200
  [for-next][PATCH 5/7] rcu: Fix dyntick-idle tracing Steven Rostedt <rostedt@goodmis.org> - 2017-04-10 22:00 +0200
  [for-next][PATCH 1/7] ftrace: Add use of synchronize_rcu_tasks() with dynamic trampolines Steven Rostedt <rostedt@goodmis.org> - 2017-04-10 22:00 +0200
  [for-next][PATCH 3/7] tracing: Add stack_tracer_disable/enable() functions Steven Rostedt <rostedt@goodmis.org> - 2017-04-10 22:00 +0200
  [for-next][PATCH 2/7] tracing: Replace the per_cpu() with __this_cpu*() in trace_stack.c Steven Rostedt <rostedt@goodmis.org> - 2017-04-10 22:00 +0200

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