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

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Message-ID: <20081113171220.GX3810@one.firstfloor.org>
Date:	Thu, 13 Nov 2008 18:12:20 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Frédéric Weisbecker <fweisbec@...il.com>
Cc:	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless

> > 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.

-Andi
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