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

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Date:	Wed, 20 Aug 2008 09:43:34 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Eran Liberty <liberty@...ricom.com>
cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"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>,
	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, Eran Liberty wrote:

> Steven Rostedt wrote:
> > 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
> > 
> > 
> > 
> >   
> powerpc-linux-gnu-objdump -dr --start-address=0xc00bb584 vmlinux | head -n 100
> 
> vmlinux:     file format elf32-powerpc
> 
> Disassembly of section .text:
> 
> c00bb584 <__d_lookup>:

[...]

> c00bb670:       41 9e 00 50     beq-    cr7,c00bb6c0 <__d_lookup+0x13c>
> c00bb674:       83 de 00 00     lwz     r30,0(r30)
> c00bb678:       2f 9e 00 00     cmpwi   cr7,r30,0
> c00bb67c:       40 9e ff 98     bne+    cr7,c00bb614 <__d_lookup+0x90>
> c00bb680:       38 60 00 00     li      r3,0
> c00bb684:       81 61 00 00     lwz     r11,0(r1)
> c00bb688:       80 0b 00 04     lwz     r0,4(r11)
> c00bb68c:       7d 61 5b 78     mr      r1,r11

[ BUG HERE IF INTERRUPT HAPPENS ]

> c00bb690:       bb 0b ff e0     lmw     r24,-32(r11)
> c00bb694:       7c 08 03 a6     mtlr    r0
> c00bb698:       4e 80 00 20     blr

Yep, you have the same bug in your compiler.

-- Steve


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