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

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Date:   Wed, 10 Apr 2019 21:34:25 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [RFC patch 16/41] tracing: Remove the ULONG_MAX stack trace
 hackery

On Wed, Apr 10, 2019 at 12:28:10PM +0200, Thomas Gleixner wrote:
> No architecture terminates the stack trace with ULONG_MAX anymore. As the
> code checks the number of entries stored anyway there is no point in
> keeping all that ULONG_MAX magic around.
> 
> The histogram code zeroes the storage before saving the stack, so if the
> trace is shorter than the maximum number of entries it can terminate the
> print loop if a zero entry is detected.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> ---
>  kernel/trace/trace_events_hist.c |    2 +-
>  kernel/trace/trace_stack.c       |   20 +++++---------------
>  2 files changed, 6 insertions(+), 16 deletions(-)
> 
> --- a/kernel/trace/trace_events_hist.c
> +++ b/kernel/trace/trace_events_hist.c
> @@ -5246,7 +5246,7 @@ static void hist_trigger_stacktrace_prin
>  	unsigned int i;
>  
>  	for (i = 0; i < max_entries; i++) {
> -		if (stacktrace_entries[i] == ULONG_MAX)
> +		if (!stacktrace_entries[i])
>  			return;
>  
>  		seq_printf(m, "%*c", 1 + spaces, ' ');
> --- 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.

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],
};

-- 
Josh