Back in 3.16 the ftrace code was redesigned and cleaned up to remove the

double iteration list (one for registered ftrace ops, and one for
 registered "global" ops), to just use one list. That simplified the code
 but also broke the function tracing filtering on pid.
 
 This updates the code to handle the filtering again with the new logic.
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Merge tag 'trace-v4.2-rc2-fix3' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace

Pull ftrace fix from Steven Rostedt:
 "Back in 3.16 the ftrace code was redesigned and cleaned up to remove
  the double iteration list (one for registered ftrace ops, and one for
  registered "global" ops), to just use one list.  That simplified the
  code but also broke the function tracing filtering on pid.

  This updates the code to handle the filtering again with the new
  logic"

* tag 'trace-v4.2-rc2-fix3' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace:
  ftrace: Fix breakage of set_ftrace_pid

include/linux/ftrace.h

@@ -116,6 +116,7 @@ ftrace_func_t ftrace_ops_get_func(struct ftrace_ops *ops);
  *            SAVE_REGS. If another ops with this flag set is already registered
  *            for any of the functions that this ops will be registered for, then
  *            this ops will fail to register or set_filter_ip.
+ * PID     - Is affected by set_ftrace_pid (allows filtering on those pids)
  */
 enum {
 	FTRACE_OPS_FL_ENABLED			= 1 << 0,
@@ -132,6 +133,7 @@ enum {
 	FTRACE_OPS_FL_MODIFYING			= 1 << 11,
 	FTRACE_OPS_FL_ALLOC_TRAMP		= 1 << 12,
 	FTRACE_OPS_FL_IPMODIFY			= 1 << 13,
+	FTRACE_OPS_FL_PID			= 1 << 14,
 };
 
 #ifdef CONFIG_DYNAMIC_FTRACE
@@ -159,6 +161,7 @@ struct ftrace_ops {
 	struct ftrace_ops		*next;
 	unsigned long			flags;
 	void				*private;
+	ftrace_func_t			saved_func;
 	int __percpu			*disabled;
 #ifdef CONFIG_DYNAMIC_FTRACE
 	int				nr_trampolines;

kernel/trace/ftrace.c

@@ -98,6 +98,13 @@ struct ftrace_pid {
 	struct pid *pid;
 };
 
+static bool ftrace_pids_enabled(void)
+{
+	return !list_empty(&ftrace_pids);
+}
+
+static void ftrace_update_trampoline(struct ftrace_ops *ops);
+
 /*
  * ftrace_disabled is set when an anomaly is discovered.
  * ftrace_disabled is much stronger than ftrace_enabled.
@@ -109,7 +116,6 @@ static DEFINE_MUTEX(ftrace_lock);
 static struct ftrace_ops *ftrace_control_list __read_mostly = &ftrace_list_end;
 static struct ftrace_ops *ftrace_ops_list __read_mostly = &ftrace_list_end;
 ftrace_func_t ftrace_trace_function __read_mostly = ftrace_stub;
-ftrace_func_t ftrace_pid_function __read_mostly = ftrace_stub;
 static struct ftrace_ops global_ops;
 static struct ftrace_ops control_ops;
 
@@ -183,14 +189,7 @@ static void ftrace_pid_func(unsigned long ip, unsigned long parent_ip,
 	if (!test_tsk_trace_trace(current))
 		return;
 
-	ftrace_pid_function(ip, parent_ip, op, regs);
+	op->saved_func(ip, parent_ip, op, regs);
-}
-
-static void set_ftrace_pid_function(ftrace_func_t func)
-{
-	/* do not set ftrace_pid_function to itself! */
-	if (func != ftrace_pid_func)
-		ftrace_pid_function = func;
 }
 
 /**
@@ -202,7 +201,6 @@ static void set_ftrace_pid_function(ftrace_func_t func)
 void clear_ftrace_function(void)
 {
 	ftrace_trace_function = ftrace_stub;
-	ftrace_pid_function = ftrace_stub;
 }
 
 static void control_ops_disable_all(struct ftrace_ops *ops)
@@ -436,6 +434,12 @@ static int __register_ftrace_function(struct ftrace_ops *ops)
 	} else
 		add_ftrace_ops(&ftrace_ops_list, ops);
 
+	/* Always save the function, and reset at unregistering */
+	ops->saved_func = ops->func;
+
+	if (ops->flags & FTRACE_OPS_FL_PID && ftrace_pids_enabled())
+		ops->func = ftrace_pid_func;
+
 	ftrace_update_trampoline(ops);
 
 	if (ftrace_enabled)
@@ -463,15 +467,28 @@ static int __unregister_ftrace_function(struct ftrace_ops *ops)
 	if (ftrace_enabled)
 		update_ftrace_function();
 
+	ops->func = ops->saved_func;
+
 	return 0;
 }
 
 static void ftrace_update_pid_func(void)
 {
+	bool enabled = ftrace_pids_enabled();
+	struct ftrace_ops *op;
+
 	/* Only do something if we are tracing something */
 	if (ftrace_trace_function == ftrace_stub)
 		return;
 
+	do_for_each_ftrace_op(op, ftrace_ops_list) {
+		if (op->flags & FTRACE_OPS_FL_PID) {
+			op->func = enabled ? ftrace_pid_func :
+				op->saved_func;
+			ftrace_update_trampoline(op);
+		}
+	} while_for_each_ftrace_op(op);
+
 	update_ftrace_function();
 }
 
@@ -1133,7 +1150,8 @@ static struct ftrace_ops global_ops = {
 	.local_hash.filter_hash		= EMPTY_HASH,
 	INIT_OPS_HASH(global_ops)
 	.flags				= FTRACE_OPS_FL_RECURSION_SAFE |
-					  FTRACE_OPS_FL_INITIALIZED,
+					  FTRACE_OPS_FL_INITIALIZED |
+					  FTRACE_OPS_FL_PID,
 };
 
 /*
@@ -5023,7 +5041,9 @@ static void ftrace_update_trampoline(struct ftrace_ops *ops)
 
 static struct ftrace_ops global_ops = {
 	.func			= ftrace_stub,
-	.flags			= FTRACE_OPS_FL_RECURSION_SAFE | FTRACE_OPS_FL_INITIALIZED,
+	.flags			= FTRACE_OPS_FL_RECURSION_SAFE |
+				  FTRACE_OPS_FL_INITIALIZED |
+				  FTRACE_OPS_FL_PID,
 };
 
 static int __init ftrace_nodyn_init(void)
@@ -5080,11 +5100,6 @@ void ftrace_init_array_ops(struct trace_array *tr, ftrace_func_t func)
 		if (WARN_ON(tr->ops->func != ftrace_stub))
 			printk("ftrace ops had %pS for function\n",
 			       tr->ops->func);
-		/* Only the top level instance does pid tracing */
-		if (!list_empty(&ftrace_pids)) {
-			set_ftrace_pid_function(func);
-			func = ftrace_pid_func;
-		}
 	}
 	tr->ops->func = func;
 	tr->ops->private = tr;
@@ -5371,7 +5386,7 @@ static void *fpid_start(struct seq_file *m, loff_t *pos)
 {
 	mutex_lock(&ftrace_lock);
 
-	if (list_empty(&ftrace_pids) && (!*pos))
+	if (!ftrace_pids_enabled() && (!*pos))
 		return (void *) 1;
 
 	return seq_list_start(&ftrace_pids, *pos);
@@ -5610,6 +5625,7 @@ static struct ftrace_ops graph_ops = {
 	.func			= ftrace_stub,
 	.flags			= FTRACE_OPS_FL_RECURSION_SAFE |
 				   FTRACE_OPS_FL_INITIALIZED |
+				   FTRACE_OPS_FL_PID |
 				   FTRACE_OPS_FL_STUB,
 #ifdef FTRACE_GRAPH_TRAMP_ADDR
 	.trampoline		= FTRACE_GRAPH_TRAMP_ADDR,