[PATCH] tracing/fgraph: remove unnecessary unlikely()

[PATCH] tracing/fgraph: remove unnecessary unlikely()

[Yangtao Li]

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

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
---
 kernel/trace/trace_functions_graph.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
index 169b3c44ee97..f8c2a08e4985 100644
--- a/kernel/trace/trace_functions_graph.c
+++ b/kernel/trace/trace_functions_graph.c
@@ -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)) {
 		ftrace_graph_stop();
-		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;
 	}
-- 
2.17.0

Re: [PATCH] tracing/fgraph: remove unnecessary unlikely()

[Steven Rostedt]

On Sat,  3 Nov 2018 22:35:37 -0400
Yangtao Li <tiny.windzz@gmail.com> wrote:

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

NACK... see below.

> 
> Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
> ---
>  kernel/trace/trace_functions_graph.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
> index 169b3c44ee97..f8c2a08e4985 100644
> --- a/kernel/trace/trace_functions_graph.c
> +++ b/kernel/trace/trace_functions_graph.c
> @@ -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.

>  		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


> -		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;
>  	}

Re: [PATCH] tracing/fgraph: remove unnecessary unlikely()

[Frank Lee]

Sorry for this place, I should consider this situation.
-- Yangtao

On Sun, Nov 4, 2018 at 10:06 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Sat,  3 Nov 2018 22:35:37 -0400
> Yangtao Li <tiny.windzz@gmail.com> wrote:
>
> > WARN_ON() already contains an unlikely(), so it's not necessary to use
> > unlikely.
>
> NACK... see below.
>
> >
> > Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
> > ---
> >  kernel/trace/trace_functions_graph.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
> > index 169b3c44ee97..f8c2a08e4985 100644
> > --- a/kernel/trace/trace_functions_graph.c
> > +++ b/kernel/trace/trace_functions_graph.c
> > @@ -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.
>
> >               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
>
>
> > -             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;
> >       }
>