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

[Josh Boyer]

On Wed, 2008-08-20 at 09:14 -0400, 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?

Segher was looking at this a bit this morning.  He thinks it's really
-fno-omit-frame-pointer that is causing this.  That really shouldn't
even be set on PowerPC, but FTRACE uses select which overrides the
depends on stuff in Kconfig.

Segher can probably tell you more once his email is back up.

josh