Re: [PATCH 5/5][RFC] function-graph: add option to calculate graph time or not

Steven Rostedt
2009-03-25T17:36:49Z

On Wed, 25 Mar 2009, Ingo Molnar wrote:

>
> * Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > --- a/kernel/trace/trace_functions_graph.c
> > +++ b/kernel/trace/trace_functions_graph.c
> > @@ -104,9 +105,6 @@ ftrace_pop_return_trace(struct ftrace_graph_ret *trace, unsigned long *ret)
> > trace->calltime = current->ret_stack[index].calltime;
> > trace->overrun = atomic_read(&current->trace_overrun);
> > trace->depth = index;
> > - barrier();
> > - current->curr_ret_stack--;
> > -
> > }
> >
> > /*
> > @@ -121,6 +119,8 @@ unsigned long ftrace_return_to_handler(void)
> > ftrace_pop_return_trace(&trace, &ret);
> > trace.rettime = trace_clock_local();
> > ftrace_graph_return(&trace);
> > + barrier();
> > + current->curr_ret_stack--;
>
> this will modify the regular graph tracer too. No discussion of this
> change in the changelog - is it a fix?

Ah, sorry, I should have commented that.

This is to prevent a race in the profiler code. This is not a problem with
current function graph code.

What happens is that the profiler references the ret_stack from the
ftrace_graph_return function. If we update the curr_ret_stack and we take
an interrupt then we run the risk of reading incorrect data. Once we
decrement the curr_ret_stack, the ret_stack for the function is fair game
to be used again by an interrupt.

Perhaps I should pull this change out and add it for core. This will let
any function graph user safely reference the ret_stack.

-- Steve