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

Peter Zijlstra
2017-01-12T16:38:24Z

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@lge.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.