tracing/function-return-tracer: support for dynamic ftrace on function return tracer

This patch adds the support for dynamic tracing on the function return tracer.
The whole difference with normal dynamic function tracing is that we don't need
to hook on a particular callback. The only pro that we want is to nop or set
dynamically the calls to ftrace_caller (which is ftrace_return_caller here).

Some security checks ensure that we are not trying to launch dynamic tracing for
return tracing while normal function tracing is already running.

An example of trace with getnstimeofday set as a filter:

ktime_get_ts+0x22/0x50 -> getnstimeofday (2283 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1396 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1382 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1825 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1426 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1464 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1524 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1382 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1382 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1434 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1464 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1502 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1404 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1397 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1051 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1314 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1344 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1163 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1390 ns)
ktime_get_ts+0x22/0x50 -> getnstimeofday (1374 ns)

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

3 files changed, 55 insertions, 19 deletions

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index 9c89526..b8378fa 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -59,7 +59,6 @@ config FUNCTION_TRACER
 
 config FUNCTION_RET_TRACER
 	bool "Kernel Function return Tracer"
-	depends on !DYNAMIC_FTRACE
 	depends on HAVE_FUNCTION_RET_TRACER
 	depends on FUNCTION_TRACER
 	help
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index b42ec1d..2f78a45 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -50,6 +50,9 @@ static int last_ftrace_enabled;
 /* Quick disabling of function tracer. */
 int function_trace_stop;
 
+/* By default, current tracing type is normal tracing. */
+enum ftrace_tracing_type_t ftrace_tracing_type = FTRACE_TYPE_ENTER;
+
 /*
  * ftrace_disabled is set when an anomaly is discovered.
  * ftrace_disabled is much stronger than ftrace_enabled.
@@ -385,12 +388,21 @@ static void ftrace_bug(int failed, unsigned long ip)
 	}
 }
 
-#define FTRACE_ADDR ((long)(ftrace_caller))
 
 static int
 __ftrace_replace_code(struct dyn_ftrace *rec, int enable)
 {
 	unsigned long ip, fl;
+	unsigned long ftrace_addr;
+
+#ifdef CONFIG_FUNCTION_RET_TRACER
+	if (ftrace_tracing_type == FTRACE_TYPE_ENTER)
+		ftrace_addr = (unsigned long)ftrace_caller;
+	else
+		ftrace_addr = (unsigned long)ftrace_return_caller;
+#else
+	ftrace_addr = (unsigned long)ftrace_caller;
+#endif
 
 	ip = rec->ip;
 
@@ -450,9 +462,9 @@ __ftrace_replace_code(struct dyn_ftrace *rec, int enable)
 	}
 
 	if (rec->flags & FTRACE_FL_ENABLED)
-		return ftrace_make_call(rec, FTRACE_ADDR);
+		return ftrace_make_call(rec, ftrace_addr);
 	else
-		return ftrace_make_nop(NULL, rec, FTRACE_ADDR);
+		return ftrace_make_nop(NULL, rec, ftrace_addr);
 }
 
 static void ftrace_replace_code(int enable)
@@ -1405,10 +1417,17 @@ int register_ftrace_function(struct ftrace_ops *ops)
 		return -1;
 
 	mutex_lock(&ftrace_sysctl_lock);
+
+	if (ftrace_tracing_type == FTRACE_TYPE_RETURN) {
+		ret = -EBUSY;
+		goto out;
+	}
+
 	ret = __register_ftrace_function(ops);
 	ftrace_startup();
-	mutex_unlock(&ftrace_sysctl_lock);
 
+out:
+	mutex_unlock(&ftrace_sysctl_lock);
 	return ret;
 }
 
@@ -1474,16 +1493,45 @@ ftrace_enable_sysctl(struct ctl_table *table, int write,
 }
 
 #ifdef CONFIG_FUNCTION_RET_TRACER
+
+/* The callback that hooks the return of a function */
 trace_function_return_t ftrace_function_return =
 			(trace_function_return_t)ftrace_stub;
-void register_ftrace_return(trace_function_return_t func)
+
+int register_ftrace_return(trace_function_return_t func)
 {
+	int ret = 0;
+
+	mutex_lock(&ftrace_sysctl_lock);
+
+	/*
+	 * Don't launch return tracing if normal function
+	 * tracing is already running.
+	 */
+	if (ftrace_trace_function != ftrace_stub) {
+		ret = -EBUSY;
+		goto out;
+	}
+
+	ftrace_tracing_type = FTRACE_TYPE_RETURN;
 	ftrace_function_return = func;
+	ftrace_startup();
+
+out:
+	mutex_unlock(&ftrace_sysctl_lock);
+	return ret;
 }
 
 void unregister_ftrace_return(void)
 {
+	mutex_lock(&ftrace_sysctl_lock);
+
 	ftrace_function_return = (trace_function_return_t)ftrace_stub;
+	ftrace_shutdown();
+	/* Restore normal tracing type */
+	ftrace_tracing_type = FTRACE_TYPE_ENTER;
+
+	mutex_unlock(&ftrace_sysctl_lock);
 }
 #endif
 
diff --git a/kernel/trace/trace_functions_return.c b/kernel/trace/trace_functions_return.c
index 61185f7..a68564a 100644
--- a/kernel/trace/trace_functions_return.c
+++ b/kernel/trace/trace_functions_return.c
@@ -14,29 +14,18 @@
 #include "trace.h"
 
 
-static void start_return_trace(struct trace_array *tr)
-{
-	register_ftrace_return(&trace_function_return);
-}
-
-static void stop_return_trace(struct trace_array *tr)
-{
-	unregister_ftrace_return();
-}
-
 static int return_trace_init(struct trace_array *tr)
 {
 	int cpu;
 	for_each_online_cpu(cpu)
 		tracing_reset(tr, cpu);
 
-	start_return_trace(tr);
-	return 0;
+	return register_ftrace_return(&trace_function_return);
 }
 
 static void return_trace_reset(struct trace_array *tr)
 {
-		stop_return_trace(tr);
+		unregister_ftrace_return();
 }