linux-kernel - Re: [RFC patch 16/41] tracing: Remove the ULONG

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Date:   Wed, 10 Apr 2019 23:07:59 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [RFC patch 16/41] tracing: Remove the ULONG_MAX stack trace
 hackery

On Wed, 10 Apr 2019 21:34:25 -0500
Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> > --- a/kernel/trace/trace_stack.c
> > +++ b/kernel/trace/trace_stack.c
> > @@ -18,8 +18,7 @@
> >  
> >  #include "trace.h"
> >  
> > -static unsigned long stack_dump_trace[STACK_TRACE_ENTRIES+1] =
> > -	 { [0 ... (STACK_TRACE_ENTRIES)] = ULONG_MAX };
> > +static unsigned long stack_dump_trace[STACK_TRACE_ENTRIES + 1];  
> 
> Is the "+ 1" still needed?  AFAICT, accesses to this array never go past
> nr_entries.

Probably not. But see this for an explanation:

 http://lkml.kernel.org/r/20180620110758.crunhd5bfep7zuiz@kili.mountain


> 
> Also I've been staring at the code but I can't figure out why
> max_entries is "- 1".
> 
> struct stack_trace stack_trace_max = {
> 	.max_entries		= STACK_TRACE_ENTRIES - 1,
> 	.entries		= &stack_dump_trace[0],
> };
> 

Well, it had a reason in the past, but there doesn't seem to be a
reason today.  Looking at git history, that code was originally:

	.max_entries		= STACK_TRACE_ENTRIES - 1,
	.entries		= &stack_dump_trace[1],

Where we had to make max_entries -1 as we started at the first index
into the array.

I'll have to take a new look into this code. After Thomas's clean up
here, I'm sure we can simplify it a bit more.

-- Steve