tracing/fgraph: remove unnecessary unlikely()

WARN_ON() already contains an unlikely(), so it's not necessary to use unlikely.

Signed-off-by Yangtao Li <[email protected]>

---
 kernel/trace/trace_functions_graph.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

kernel/trace/trace_functions_graph.c +0 -0

index 169b3c44ee97..f8c2a08e4985 100644

kernel/trace/trace_functions_graph.c +1 -3

-		WARN_ON(1);
 		/* Might as well panic, otherwise we have no where to go */
 		*ret = (unsigned long)panic;
 		return;

@@ -274,9 +273,8 @@ unsigned long ftrace_return_to_handler(unsigned long frame_pointer)
 	 */
 	ftrace_graph_return(&trace);
 
-	if (unlikely(!ret)) {
+	if (WARN_ON(!ret)) {
 		ftrace_graph_stop();
-		WARN_ON(1);
 		/* Might as well panic. What else to do? */
 		ret = (unsigned long)panic;
 	}

Quoted 2 lines · On Sat, 3 Nov 2018 22:35:37 -0400 Yangtao Li <[email protected]> wrote:

WARN_ON() already contains an unlikely(), so it's not necessary to use unlikely.

NACK... see below.

Quoted 11 lines

Signed-off-by Yangtao Li <[email protected]>

---
 kernel/trace/trace_functions_graph.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

kernel/trace/trace_functions_graph.c +0 -0

index 169b3c44ee97..f8c2a08e4985 100644

kernel/trace/trace_functions_graph.c ftrace_pop_return_trace(struct ftrace_graph_ret *trace, unsigned long *ret, · line 204

Quoted 5 lines

kernel/trace/trace_functions_graph.c +1 -1

@@ -201,9 +201,8 @@ ftrace_pop_return_trace(struct ftrace_graph_ret *trace, unsigned long *ret,
      if (index < 0)
              index += FTRACE_NOTRACE_DEPTH;
 
-     if (unlikely(index < 0 || index >= FTRACE_RETFUNC_DEPTH)) {
+     if (WARN_ON(index < 0 || index >= FTRACE_RETFUNC_DEPTH)) {

When function graph is enabled, the printk from the warn on will likely be traced as well and trigger the same WARN_ON, which will again cause a recursion loop and crash and reboot the box with no output at all.

kernel/trace/trace_functions_graph.c ftrace_pop_return_trace(struct ftrace_graph_ret *trace, unsigned long *ret, · line 205

Quoted 1 line

kernel/trace/trace_functions_graph.c +0 -0

              ftrace_graph_stop();

Notice that I call ftrace_graph_stop() *before* the WARN_ON(). This disables the ftrace graph tracer and prevents the recursion loop from happening.

There's a reason that the WARN_ON() is placed where it is.

But thanks for the report, it shows that I need to add a comment here so that someone else doesn't send a similar patch in the future.

-- Steve

Quoted 15 lines

kernel/trace/trace_functions_graph.c +1 -3

-             WARN_ON(1);
              /* Might as well panic, otherwise we have no where to go */
              *ret = (unsigned long)panic;
              return;

@@ -274,9 +273,8 @@ unsigned long ftrace_return_to_handler(unsigned long frame_pointer)
       */
      ftrace_graph_return(&trace);
 
-     if (unlikely(!ret)) {
+     if (WARN_ON(!ret)) {
              ftrace_graph_stop();
-             WARN_ON(1);
              /* Might as well panic. What else to do? */
              ret = (unsigned long)panic;
      }