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

From: Eran Liberty <[email-masked]>
Date: Wed, 20 Aug 2008 16:36:24 +0300

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>:
c00bb584:       7c 08 02 a6     mflr    r0
c00bb588:       90 01 00 04     stw     r0,4(r1)
c00bb58c:       4b f5 5c 51     bl      c00111dc <_mcount>
c00bb590:       94 21 ff d0     stwu    r1,-48(r1)
c00bb594:       7c 08 02 a6     mflr    r0
c00bb598:       3d 20 9e 37     lis     r9,-25033
c00bb59c:       bf 01 00 10     stmw    r24,16(r1)
c00bb5a0:       61 29 00 01     ori     r9,r9,1
c00bb5a4:       3d 60 c0 38     lis     r11,-16328
c00bb5a8:       90 01 00 34     stw     r0,52(r1)
c00bb5ac:       7c 60 4a 78     xor     r0,r3,r9
c00bb5b0:       54 00 d9 7e     rlwinm  r0,r0,27,5,31
c00bb5b4:       83 84 00 00     lwz     r28,0(r4)
c00bb5b8:       7c 3f 0b 78     mr      r31,r1
c00bb5bc:       81 0b 1a 2c     lwz     r8,6700(r11)
c00bb5c0:       39 6b 1a 2c     addi    r11,r11,6700
c00bb5c4:       7c 00 e2 14     add     r0,r0,r28
c00bb5c8:       81 4b 00 04     lwz     r10,4(r11)
c00bb5cc:       7c 09 4a 78     xor     r9,r0,r9
c00bb5d0:       83 24 00 04     lwz     r25,4(r4)
c00bb5d4:       7d 29 44 30     srw     r9,r9,r8
c00bb5d8:       81 0b 00 08     lwz     r8,8(r11)
c00bb5dc:       7d 29 02 78     xor     r9,r9,r0
c00bb5e0:       83 04 00 08     lwz     r24,8(r4)
c00bb5e4:       7d 29 50 38     and     r9,r9,r10
c00bb5e8:       55 29 10 3a     rlwinm  r9,r9,2,0,29
c00bb5ec:       7c 09 40 2e     lwzx    r0,r9,r8
c00bb5f0:       7c 9a 23 78     mr      r26,r4
c00bb5f4:       7c 7b 1b 78     mr      r27,r3
c00bb5f8:       2f 80 00 00     cmpwi   cr7,r0,0
c00bb5fc:       7c 1e 03 78     mr      r30,r0
c00bb600:       40 be 00 14     bne+    cr7,c00bb614 <__d_lookup+0x90>
c00bb604:       48 00 00 7c     b       c00bb680 <__d_lookup+0xfc>
c00bb608:       83 de 00 00     lwz     r30,0(r30)
c00bb60c:       2f 9e 00 00     cmpwi   cr7,r30,0
c00bb610:       41 9e 00 70     beq-    cr7,c00bb680 <__d_lookup+0xfc>
c00bb614:       80 1e 00 00     lwz     r0,0(r30)
c00bb618:       2f 80 00 00     cmpwi   cr7,r0,0
c00bb61c:       41 9e 00 08     beq-    cr7,c00bb624 <__d_lookup+0xa0>
c00bb620:       7c 00 02 2c     dcbt    r0,r0
c00bb624:       3b be ff f4     addi    r29,r30,-12
c00bb628:       80 1d 00 18     lwz     r0,24(r29)
c00bb62c:       7f 80 e0 00     cmpw    cr7,r0,r28
c00bb630:       40 9e ff d8     bne+    cr7,c00bb608 <__d_lookup+0x84>
c00bb634:       80 1d 00 14     lwz     r0,20(r29)
c00bb638:       7f 80 d8 00     cmpw    cr7,r0,r27
c00bb63c:       40 9e ff cc     bne+    cr7,c00bb608 <__d_lookup+0x84>
c00bb640:       81 3b 00 48     lwz     r9,72(r27)
c00bb644:       38 9d 00 18     addi    r4,r29,24
c00bb648:       2f 89 00 00     cmpwi   cr7,r9,0
c00bb64c:       41 9e 00 50     beq-    cr7,c00bb69c <__d_lookup+0x118>
c00bb650:       80 09 00 08     lwz     r0,8(r9)
c00bb654:       2f 80 00 00     cmpwi   cr7,r0,0
c00bb658:       41 9e 00 44     beq-    cr7,c00bb69c <__d_lookup+0x118>
c00bb65c:       7f 63 db 78     mr      r3,r27
c00bb660:       7c 09 03 a6     mtctr   r0
c00bb664:       7f 45 d3 78     mr      r5,r26
c00bb668:       4e 80 04 21     bctrl
c00bb66c:       2f 83 00 00     cmpwi   cr7,r3,0
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
c00bb690:       bb 0b ff e0     lmw     r24,-32(r11)
c00bb694:       7c 08 03 a6     mtlr    r0
c00bb698:       4e 80 00 20     blr
c00bb69c:       80 04 00 04     lwz     r0,4(r4)
c00bb6a0:       7f 80 c8 00     cmpw    cr7,r0,r25
c00bb6a4:       40 9e ff 64     bne+    cr7,c00bb608 <__d_lookup+0x84>
c00bb6a8:       80 64 00 08     lwz     r3,8(r4)
c00bb6ac:       7f 25 cb 78     mr      r5,r25
c00bb6b0:       7f 04 c3 78     mr      r4,r24
c00bb6b4:       4b f5 ab 65     bl      c0016218 <memcmp>
c00bb6b8:       2f 83 00 00     cmpwi   cr7,r3,0
c00bb6bc:       40 9e ff 4c     bne+    cr7,c00bb608 <__d_lookup+0x84>
c00bb6c0:       80 1d 00 04     lwz     r0,4(r29)
c00bb6c4:       70 09 00 10     andi.   r9,r0,16
c00bb6c8:       40 a2 ff b8     bne-    c00bb680 <__d_lookup+0xfc>
c00bb6cc:       7c 00 e8 28     lwarx   r0,0,r29
c00bb6d0:       30 00 00 01     addic   r0,r0,1
c00bb6d4:       7c 00 e9 2d     stwcx.  r0,0,r29
c00bb6d8:       40 a2 ff f4     bne-    c00bb6cc <__d_lookup+0x148>
c00bb6dc:       7f a3 eb 78     mr      r3,r29
c00bb6e0:       4b ff ff a4     b       c00bb684 <__d_lookup+0x100>

c00bb6e4 <d_lookup>:
c00bb6e4:       7c 08 02 a6     mflr    r0
c00bb6e8:       90 01 00 04     stw     r0,4(r1)