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

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Message-ID: <alpine.DEB.1.10.0808181425090.796@gandalf.stny.rr.com>
Date:	Mon, 18 Aug 2008 14:27:14 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Scott Wood <scottwood@...escale.com>
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)



On Mon, 18 Aug 2008, Scott Wood wrote:

> Mathieu Desnoyers wrote:
> >         asm volatile (
> >                 "1: lwz         %1, 0(%2)\n"
> >                 "   cmpw        %1, %5\n"
> >                 "   bne         2f\n"
> >                 "   stwu        %3, 0(%2)\n"
> >                 "2:\n"
> >                 ".section .fixup, \"ax\"\n"
> >                 "3:     li %0, 1\n"
> >                 "       b 2b\n"
> >                 ".previous\n"
> >                 ".section __ex_table,\"a\"\n"
> >                 _ASM_ALIGN "\n"
> >                 _ASM_PTR "1b, 3b\n"
> >                 ".previous"
> >                 : "=r"(faulted), "=r"(replaced)
> >                 : "r"(ip), "r"(new),
> >                   "0"(faulted), "r"(old)
> >                 : "memory");
> 
> Some (most likely unrelated) nits in the above inline asm:

Thanks,

> 
> 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 ;-)

> Why stwu with an offset of zero,
> BTW?

Because it worked :-)  Really, it has been a while since I did any PPC 
assembly, and I didn't break out the old reference manuals to do this.
I simply looked at other asm code, and followed suit. I compiled and 
tested it, and it worked.  I did make a disclaimer about my rusty PPC
knowledge when I posted the code.

> 
> %1 also needs to be an early clobber.

Not exactly sure what you mean by the above.

-- Steve
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