linux-kernel - Re: [PATCH] stacktrace: don't skip first entry on noncurrent tasks

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Date:   Fri, 25 Oct 2019 09:21:10 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jiri Slaby <jslaby@...e.cz>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] stacktrace: don't skip first entry on noncurrent tasks

On Fri, Oct 25, 2019 at 08:52:26AM +0200, Jiri Slaby wrote:
> When doing cat /proc/<PID>/stack, the output is missing the first entry.
> When the current code walks the stack starting in stack_trace_save_tsk,
> it skips all scheduler functions (that's OK) plus one more function. But
> this one function should be skipped only for the 'current' task as it is
> stack_trace_save_tsk proper.
> 
> The original code (before the common infrastructure) skipped one
> function only for the 'current' task -- see save_stack_trace_tsk before
> 3599fe12a125. So do so also in the new infrastructure now.
> 
> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
> Fixes: 214d8ca6ee85 ("stacktrace: Provide common infrastructure")
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> ---
>  kernel/stacktrace.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/stacktrace.c b/kernel/stacktrace.c
> index 6d1f68b7e528..d06a2e4d0142 100644
> --- a/kernel/stacktrace.c
> +++ b/kernel/stacktrace.c
> @@ -141,7 +141,8 @@ unsigned int stack_trace_save_tsk(struct task_struct *tsk, unsigned long *store,
>  	struct stacktrace_cookie c = {
>  		.store	= store,
>  		.size	= size,
> -		.skip	= skipnr + 1,
> +		/* skip this function if they are tracing us */
> +		.skip	= skipnr + !!(current == tsk),
>  	};
>  
>  	if (!try_get_task_stack(tsk))

I think a similar change is needed for the other !CONFIG_ARCH_STACKWALK
version of stack_trace_save_tsk() in that file?

-- 
Josh