linux-kernel - Re: [PATCH -v4 9/9] tracing: add function graph tracer support for MIPS

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Message-ID: <4ADF5225.40701@caviumnetworks.com>
Date:	Wed, 21 Oct 2009 11:25:41 -0700
From:	David Daney <ddaney@...iumnetworks.com>
To:	rostedt@...dmis.org
CC:	Wu Zhangjin <wuzhangjin@...il.com>, linux-kernel@...r.kernel.org,
	linux-mips@...ux-mips.org, Thomas Gleixner <tglx@...utronix.de>,
	Ralf Baechle <ralf@...ux-mips.org>,
	Nicholas Mc Guire <der.herr@...r.at>
Subject: Re: [PATCH -v4 9/9] tracing: add function graph tracer support for
 MIPS

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
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