[PATCH 0/5 v2] tracing: Add usecase of synchronize_rcu_tasks() and stack_tracer_disable()

[Steven Rostedt]

Paul,

Here's my latest. You OK with it?

-- Steve


Paul E. McKenney (1):
      rcu: Fix dyntick-idle tracing

Steven Rostedt (VMware) (4):
      ftrace: Add use of synchronize_rcu_tasks() with dynamic trampolines
      tracing: Replace the per_cpu() with this_cpu() in trace_stack.c
      tracing: Add stack_tracer_disable/enable() functions
      tracing: Rename trace_active to disable_stack_tracer and inline its modification

----
Changes since v1:

  . Make stack_tracer_{dis,en}able() into inlines

  . Rename trace_active to disable_stack_tracer and make global

  . Do not call preempt_disable() from stack_tracer_disable()
    and likewise from stack_tracer_enable()

  . Add check for preemption disabled when CONFIG_PREEMPT_DEBUG

 include/linux/ftrace.h     | 38 ++++++++++++++++++++++++++++++++++++
 kernel/rcu/tree.c          | 48 ++++++++++++++++++++++------------------------
 kernel/trace/Kconfig       |  3 ++-
 kernel/trace/ftrace.c      | 42 +++++++++++++++++-----------------------
 kernel/trace/trace_stack.c | 27 +++++++++-----------------
 5 files changed, 90 insertions(+), 68 deletions(-)

Diff from v1:

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index 40afee3..4bde7ff 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -287,8 +287,40 @@ 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);
+/* DO NOT MODIFY THIS VARIABLE DIRECTLY! */
+DECLARE_PER_CPU(int, disable_stack_tracer);
+
+/**
+ * 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.
+ */
+static inline 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(disable_stack_tracer);
+}
+
+/**
+ * stack_tracer_enable - re-enable the stack tracer
+ *
+ * After stack_tracer_disable() is called, stack_tracer_enable()
+ * must be called shortly afterward.
+ */
+static inline void stack_tracer_enable(void)
+{
+	if (IS_ENABLED(CONFIG_PREEMPT_DEBUG))
+		WARN_ON_ONCE(!preempt_count() || !irqs_disabled());
+	this_cpu_dec(disable_stack_tracer);
+}
 #else
 static inline void stack_tracer_disable(void) { }
 static inline void stack_tracer_enabe(void) { }
diff --git a/kernel/trace/trace_stack.c b/kernel/trace/trace_stack.c
index 5adbb73..84fafb6 100644
--- a/kernel/trace/trace_stack.c
+++ b/kernel/trace/trace_stack.c
@@ -35,40 +35,12 @@ unsigned long stack_trace_max_size;
 arch_spinlock_t stack_trace_max_lock =
 	(arch_spinlock_t)__ARCH_SPIN_LOCK_UNLOCKED;
 
-static DEFINE_PER_CPU(int, trace_active);
+DEFINE_PER_CPU(int, disable_stack_tracer);
 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;
@@ -239,8 +211,8 @@ stack_trace_call(unsigned long ip, unsigned long parent_ip,
 	preempt_disable_notrace();
 
 	/* no atomic needed, we only modify this variable by this cpu */
-	this_cpu_inc(trace_active);
-	if (this_cpu_read(trace_active) != 1)
+	this_cpu_inc(disable_stack_tracer);
+	if (this_cpu_read(disable_stack_tracer) != 1)
 		goto out;
 
 	ip += MCOUNT_INSN_SIZE;
@@ -248,7 +220,7 @@ stack_trace_call(unsigned long ip, unsigned long parent_ip,
 	check_stack(ip, &stack);
 
  out:
-	this_cpu_dec(trace_active);
+	this_cpu_dec(disable_stack_tracer);
 	/* prevent recursion in schedule */
 	preempt_enable_notrace();
 }
@@ -290,15 +262,15 @@ stack_max_size_write(struct file *filp, const char __user *ubuf,
 	/*
 	 * In case we trace inside arch_spin_lock() or after (NMI),
 	 * we will cause circular lock, so we also need to increase
-	 * the percpu trace_active here.
+	 * the percpu disable_stack_tracer here.
 	 */
-	this_cpu_inc(trace_active);
+	this_cpu_inc(disable_stack_tracer);
 
 	arch_spin_lock(&stack_trace_max_lock);
 	*ptr = val;
 	arch_spin_unlock(&stack_trace_max_lock);
 
-	this_cpu_dec(trace_active);
+	this_cpu_dec(disable_stack_tracer);
 	local_irq_restore(flags);
 
 	return count;
@@ -334,7 +306,7 @@ static void *t_start(struct seq_file *m, loff_t *pos)
 {
 	local_irq_disable();
 
-	this_cpu_inc(trace_active);
+	this_cpu_inc(disable_stack_tracer);
 
 	arch_spin_lock(&stack_trace_max_lock);
 
@@ -348,7 +320,7 @@ static void t_stop(struct seq_file *m, void *p)
 {
 	arch_spin_unlock(&stack_trace_max_lock);
 
-	this_cpu_dec(trace_active);
+	this_cpu_dec(disable_stack_tracer);
 
 	local_irq_enable();
 }