linux-kernel - Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)

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Date:	Mon, 18 Aug 2008 13:29:48 -0500
From:	Scott Wood <scottwood@...escale.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Steven Rostedt <srostedt@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Eran Liberty <liberty@...ricom.com>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org
Subject: Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)

Steven Rostedt wrote:
>> Should use a "b" constraint for %2, or you could get r0. 
> 
> I will make an updated patch.
> 
>> Or, use an "m"
>> constraint with %U2%X2 after the lwz/stw. 
> 
> The 'b' seems easier ;-)

The advantage of the latter is that it allows GCC to choose indexed or 
update instructions -- but that's merely an optimization.  Switching to 
"b" is enough to avoid the potential bug.

>> %1 also needs to be an early clobber.
> 
> Not exactly sure what you mean by the above.

%1 is written to before some inputs are consumed, so you need to use 
"=&r" rather than "=r" so that GCC won't use the same register for both.

-Scott
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