linux-kernel - Re: [PATCH v4 06/15] lockdep: Make save_trace can skip stack tracing of the current

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Date:   Thu, 12 Jan 2017 17:37:57 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Byungchul Park <byungchul.park@....com>
Cc:     mingo@...nel.org, tglx@...utronix.de, walken@...gle.com,
        boqun.feng@...il.com, kirill@...temov.name,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        npiggin@...il.com
Subject: Re: [PATCH v4 06/15] lockdep: Make save_trace can skip stack tracing
 of the current

On Fri, Dec 09, 2016 at 02:12:02PM +0900, Byungchul Park wrote:
> Currently, save_trace() always performs save_stack_trace() for the
> current. However, crossrelease needs to use stack trace data of another
> context instead of the current. So add a parameter for skipping stack
> tracing of the current and make it use trace data, which is already
> saved by crossrelease framework.
> 
> Signed-off-by: Byungchul Park <byungchul.park@....com>
> ---
>  kernel/locking/lockdep.c | 33 ++++++++++++++++++++-------------
>  1 file changed, 20 insertions(+), 13 deletions(-)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 3eaa11c..11580ec 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -387,15 +387,22 @@ static void print_lockdep_off(const char *bug_msg)
>  #endif
>  }
>  
> -static int save_trace(struct stack_trace *trace)
> +static int save_trace(struct stack_trace *trace, int skip_tracing)
>  {
> -	trace->nr_entries = 0;
> -	trace->max_entries = MAX_STACK_TRACE_ENTRIES - nr_stack_trace_entries;
> -	trace->entries = stack_trace + nr_stack_trace_entries;
> +	unsigned int nr_avail = MAX_STACK_TRACE_ENTRIES - nr_stack_trace_entries;
>  
> -	trace->skip = 3;
> -
> -	save_stack_trace(trace);
> +	if (skip_tracing) {
> +		trace->nr_entries = min(trace->nr_entries, nr_avail);
> +		memcpy(stack_trace + nr_stack_trace_entries, trace->entries,
> +				trace->nr_entries * sizeof(trace->entries[0]));
> +		trace->entries = stack_trace + nr_stack_trace_entries;
> +	} else {
> +		trace->nr_entries = 0;
> +		trace->max_entries = nr_avail;
> +		trace->entries = stack_trace + nr_stack_trace_entries;
> +		trace->skip = 3;
> +		save_stack_trace(trace);
> +	}
>  
>  	/*
>  	 * Some daft arches put -1 at the end to indicate its a full trace.

That's pretty nasty semantics.. so when skip_tracing it modifies trace
in-place.