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Date: Wed, 20 Aug 2008 09:14:09 -0400 (EDT)
From: Steven Rostedt <rostedt@goodmis.org>
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To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
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       Steven Rostedt <srostedt@redhat.com>,
       Scott Wood <scottwood@freescale.com>,
       Eran Liberty <liberty@extricom.com>, Alan Modra <amodra@au1.ibm.com>,
       Segher Boessenkool <segher@kernel.crashing.org>
Subject: Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)
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On Wed, 20 Aug 2008, Benjamin Herrenschmidt wrote:

> Found the problem (or at least -a- problem), it's a gcc bug.
> 
> Well, first I must say the code generated by -pg is just plain
> horrible :-)
> 
> Appart from that, look at the exit of, for example, __d_lookup, as
> generated by gcc when ftrace is enabled:
> 
> c00c0498:       38 60 00 00     li      r3,0
> c00c049c:       81 61 00 00     lwz     r11,0(r1)
> c00c04a0:       80 0b 00 04     lwz     r0,4(r11)
> c00c04a4:       7d 61 5b 78     mr      r1,r11
> c00c04a8:       bb 0b ff e0     lmw     r24,-32(r11)
> c00c04ac:       7c 08 03 a6     mtlr    r0
> c00c04b0:       4e 80 00 20     blr
> 
> As you can see, it restores r1 -before- it pops r24..r31 off
> the stack ! I let you imagine what happens if an interrupt happens
> just in between those two instructions (mr and lmw). We don't do
> redzones on our ABI, so basically, the registers end up corrupted
> by the interrupt.

Ouch!  You've disassembled this without -pg too, and it does not have this 
bug? What version of gcc do you have?

-- Steve