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

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



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