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

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Message-ID: <20081113191931.GB3810@one.firstfloor.org>
Date:	Thu, 13 Nov 2008 20:19:31 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	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

> barrier:
> 	__asm__ __volatile__("": : :"memory")
> 
> local_add_return:
> 	asm volatile(_ASM_XADD "%0, %1;"
> 		     : "+r" (i), "+m" (l->a.counter)
> 		     : : "memory");
> 
> 
> Your argument was that barrier clobbers memory, but it looks like the 
> local_add_return does the same.

That's a fair point. I'm not sure why the local_add_* has a memory
barrier at all, imho it shouldn't need it. I bet it's just
a hold over from the atomic code this was based one.

> I don't see any major gain in switching to local_add_return.

That's true with the current implementation. It would
be a good idea to consider dropping that explicit 
barrier though, then it would be likely a slight win.

-Andi

-- 
ak@...ux.intel.com
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