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Date:	Wed, 21 Oct 2009 11:25:41 -0700
From:	David Daney <ddaney@caviumnetworks.com>
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To:	rostedt@goodmis.org
CC:	Wu Zhangjin <wuzhangjin@gmail.com>, linux-kernel@vger.kernel.org,
	linux-mips@linux-mips.org, Thomas Gleixner <tglx@linutronix.de>,
	Ralf Baechle <ralf@linux-mips.org>,
	Nicholas Mc Guire <der.herr@hofr.at>
Subject: Re: [PATCH -v4 9/9] tracing: add function graph tracer support for
 MIPS
References: <028867b99ec532b84963a35e7d552becc783cafc.1256135456.git.wuzhangjin@gmail.com>	 <3f0d3515f74a58f4cfd11e61b62a129fdc21e3a7.1256135456.git.wuzhangjin@gmail.com>	 <ea8aa927fbd184b54941e4c2ae0be8ea0b4f6b8a.1256135456.git.wuzhangjin@gmail.com>	 <96110ea5dd4d3d54eb97d0bb708a5bd81c7a50b5.1256135456.git.wuzhangjin@gmail.com>	 <5dda13e8e3a9c9dba4bb7179183941bda502604f.1256135456.git.wuzhangjin@gmail.com>	 <af3ec1b5cd06b6f6a461c9fa7d09a51fabccb08d.1256135456.git.wuzhangjin@gmail.com>	 <a6f2959a69b6a77dd32cc36a5c8202f97d524f1e.1256135456.git.wuzhangjin@gmail.com>	 <53bdfdd95ec4fa00d4cc505bb5972cf21243a14d.1256135456.git.wuzhangjin@gmail.com>	 <1256141540.18347.3118.camel@gandalf.stny.rr.com>	 <4ADF38D5.9060100@caviumnetworks.com>	 <1256143568.18347.3169.camel@gandalf.stny.rr.com>	 <4ADF3FE0.5090104@caviumnetworks.com>	 <1256145813.18347.3210.camel@gandalf.stny.rr.com>	 <4ADF4982.9010306@caviumnetworks.com> <1256148562.18347.3264.camel@gandalf.stny.rr.com>
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Steven Rostedt wrote:
[...]
> We're not doing back traces. We need to modify the return of the
> function being called. Note, the above functions that end with ";" are
> leaf functions. Non leaf functions show "{" and end with "}".
> 
> The trick here is to find a reliable way to modify the return address.
> 

Well that's what I get for chiming in without fully understanding the 
context.

I think it is fine to search backwards from the ($ra - 8) of the call to 
_mcount until you encounter the adjustment of $sp.  If you encounter a 
store of $ra, you can assume that the function will use the stored value 
when returning, otherwise modify the value restored to $ra (passed in 
$at).  That is probably what you were doing to begin with.

David Daney