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 12:32:23 -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:

> > So the answer to this is:
> > 
> >   i = index++;
> >   barrier();
> >   write to index i (not index);
> 
> That was my first thought when I wrote the original email,
> but the disadvantage is that barrier() is a big hammer
> that flushes everything and can make the code much worse.
> That is why I suggested local_add_return() instead.

barrier() is a compiler barrier, does nothing with the caches, and is 
quite cheap. We only need a compiler barrier because we are only 
protecting ourselves from things that happen on the current CPU. No other 
devices or other CPUs are involved.

-- Steve

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