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

From: Scott Wood <[email-masked]>
Date: Mon, 18 Aug 2008 13:29:48 -0500

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