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

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Date:	Wed, 25 Mar 2009 13:35:43 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Tim Bird <tim.bird@...sony.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 5/5][RFC] function-graph: add option to calculate graph
 time or not



On Wed, 25 Mar 2009, Ingo Molnar wrote:

> 
> * Steven Rostedt <rostedt@...dmis.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

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