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

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Date:	Wed, 20 Aug 2008 09:19:15 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	Steven Rostedt <srostedt@...hat.com>,
	Scott Wood <scottwood@...escale.com>,
	Eran Liberty <liberty@...ricom.com>,
	Alan Modra <amodra@....ibm.com>,
	Segher Boessenkool <segher@...nel.crashing.org>
Subject: Re: ftrace introduces instability into kernel 2.6.27(-rc2,-rc3)


On Wed, 20 Aug 2008, Steven Rostedt wrote:

> 
> 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?
> 

I have:
 gcc (Debian 4.3.1-2) 4.3.1

c00c64c8:       81 61 00 00     lwz     r11,0(r1)
c00c64cc:       7f 83 e3 78     mr      r3,r28
c00c64d0:       80 0b 00 04     lwz     r0,4(r11)
c00c64d4:       ba eb ff dc     lmw     r23,-36(r11)
c00c64d8:       7d 61 5b 78     mr      r1,r11
c00c64dc:       7c 08 03 a6     mtlr    r0
c00c64e0:       4e 80 00 20     blr


My version looks fine.  I'm thinking that this is a separate issue than 
what Eran is seeing.

Eran, can you do an "objdump -dr vmlinux" and search for __d_lookup, and 
print out the end of the function dump.

Thanks,

-- Steve

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