[FTRACE] Enabling function_graph causes OOPS

[Sachin Sant]

While enabling function_graph tracer on a Power6 box, machine
crashed with following trace. Kernel version is 2.6.31-rc3.

:/debug/tracing # echo function_graph > current_tracer 
:/debug/tracing # cat current_tracer 
function_graph
:/debug/tracing # echo 1 > tracing_enabled

cpu 0x0: Vector: 300 (Data Access) at [c00000003eb86de0]
    pc: c000000000008f30: .mod_return_to_handler+0x2c/0x64
    lr: c000000000008f04: .mod_return_to_handler+0x0/0x64
    sp: c00000003eb87060
   msr: 8000000000009032
   dar: 8
 dsisr: 40000000
  current = 0xc00000003e83c080
  paca    = 0xc000000000ff2400
    pid   = 2700, comm = sshd
enter ? for help
[c00000003eb870d0] c000000000008f04 .mod_return_to_handler+0x0/0x64
[c00000003eb871a0] c000000000008ed0 .return_to_handler+0x0/0x34
[c00000003eb87290] c000000000008ed0 .return_to_handler+0x0/0x34
[c00000003eb87330] c000000000008ed0 .return_to_handler+0x0/0x34
[c00000003eb873e0] c000000000008ed0 .return_to_handler+0x0/0x34
[c00000003eb87470] c000000000008ed0 .return_to_handler+0x0/0x34
[c00000003eb87500] c000000000008ed0 .return_to_handler+0x0/0x34
[c00000003eb87640] c000000000008ed0 .return_to_handler+0x0/0x34
[c00000003eb87730] c000000000008ed0 .return_to_handler+0x0/0x34
[c00000003eb87830] c000000000008ed0 .return_to_handler+0x0/0x34
[c00000003eb878d0] c000000000008ed0 .return_to_handler+0x0/0x34
[c00000003eb87a00] c000000000008ed0 .return_to_handler+0x0/0x34
[c00000003eb87b30] c000000000008ed0 .return_to_handler+0x0/0x34
[c00000003eb87cd0] c000000000008ed0 .return_to_handler+0x0/0x34
[c00000003eb87d80] c000000000008ed0 .return_to_handler+0x0/0x34
[c00000003eb87e30] c000000000008ed0 .return_to_handler+0x0/0x34
--- Exception: c01 (System Call) at 00000fffb640a8f8
SP (fffe5b9b960) is in userspace
0:mon> di c000000000008f04
c000000000008f04  f881ffe0      std     r4,-32(r1)
c000000000008f08  f861ffe8      std     r3,-24(r1)
c000000000008f0c  f841fff0      std     r2,-16(r1)
c000000000008f10  fbe1fff8      std     r31,-8(r1)
c000000000008f14  7c3f0b78      mr      r31,r1
c000000000008f18  f821ff91      stdu    r1,-112(r1)
c000000000008f1c  3c800000      lis     r4,0
c000000000008f20  60840000      ori     r4,r4,0
c000000000008f24  788407c6      rldicr  r4,r4,32,31
c000000000008f28  64840000      oris    r4,r4,0
c000000000008f2c  60840000      ori     r4,r4,0
c000000000008f30  e8440008      ld      r2,8(r4)  
^^^^^ PC points to this ^^^^^^^
c000000000008f34  48123431      bl      c00000000012c364        # .ftrace_return_to_handler+0x0/0x110
c000000000008f38  60000000      nop
c000000000008f3c  7c6803a6      mtlr    r3
c000000000008f40  e8210000      ld      r1,0(r1)
0:xmon> r
R00 = c000000000008f04   R16 = 00000fffb741d6a0
R01 = c00000003ea4b7e0   R17 = 00000fffb7406c38
R02 = d0000000010bec80   R18 = c00000003de41838
R03 = 0000000000000000   R19 = fffffffffffff000
R04 = 0000000000000000   R20 = c00000003cfc8c80
R05 = 0000000066000000   R21 = c00000003ea4bdf8
R06 = 00000000000010bf   R22 = c00000003ea4bba8
R07 = 0000000000000000   R23 = 00000fff8eb60000
R08 = 0000000000000000   R24 = fffffffffffff000
R09 = 800000010021c740   R25 = 8001000003000000
R10 = 00000000000000ff   R26 = ffe0000000000000
R11 = 800000010021c760   R27 = 0000000000000009
R12 = 0000000000000002   R28 = c00000006f850000
R13 = c000000000ff2400   R29 = 0000000000020000
R14 = 0000000000000000   R30 = d0000000010bd9c8
R15 = ffffffffeffdff68   R31 = c00000003ea4b850
pc  = c000000000008f30 .mod_return_to_handler+0x2c/0x64
lr  = c000000000008f04 .mod_return_to_handler+0x0/0x64
msr = 8000000000009032   cr  = 24000442
ctr = 80000000000f6d68   xer = 0000000000000020   trap =  300
dar = 0000000000000008   dsisr = 40000000
0:xmon>

Following tracers are supported by the kernel.
:/debug/tracing # cat available_tracers 
function_graph function sched_switch nop

Other tracers function and sched_switch work fine. Having problem
only with function_graph.

Have attached the .config.

Thanks
-Sachin


-- 

---------------------------------
Sachin Sant
IBM Linux Technology Center
India Systems and Technology Labs
Bangalore, India
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