linux-kernel - Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless

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Date:	Thu, 13 Nov 2008 12:21:00 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Frédéric Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] tracing/function-return-tracer: Make the function
 return tracer lockless


On Thu, 13 Nov 2008, Andi Kleen wrote:

> > > local register i
> > > i = index;
> > > write to index'ed array using i
> > >                            <--------- interrupt here would overwrite data
> > > ...
> > > index = i + 1;
> > 
> > 
> > Yes in the common case that would be a danger. But here, if an
> > interrupt is raised, it will increment
> > the counter and then decrement it at return time without dropping the
> > cpu. So after the interrupt, the
> > value will remain the same...
> 
> The buffer contents will not be necessarily
> the same. See the scenario above. The interrupt would use the 
> same i as the current function and would overwrite the
> partially written entry.


So the answer to this is:

  i = index++;
  barrier();
  write to index i (not index);

-- Steve


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