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

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Date:   Sun, 4 Nov 2018 22:32:27 +0800
From:   Frank Lee <tiny.windzz@...il.com>
To:     rostedt@...dmis.org
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tracing/fgraph: remove unnecessary unlikely()

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

On Sun, Nov 4, 2018 at 10:06 PM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> On Sat,  3 Nov 2018 22:35:37 -0400
> Yangtao Li <tiny.windzz@...il.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@...il.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;
> >       }
>