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Message-ID: <alpine.DEB.1.10.0808200916090.13132@gandalf.stny.rr.com>
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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