When an interrupt occurs in userspace, we can call trace_hardirqs_on/off()
    With one level stack. But if we have irqsoff tracing enabled,
    it checks both CALLER_ADDR0 and CALLER_ADDR1. The second call
    goes two stack frames up. If this is from user space, then there may
    not exist a second stack.

    Add a second stack when calling trace_hardirqs_on/off() otherwise
    the following oops might occur:

    Oops: Kernel access of bad area, sig: 11 [#1]
    PREEMPT SMP NR_CPUS=2 PA Semi PWRficient
    last sysfs file: /sys/block/sda/size
    Modules linked in: ohci_hcd ehci_hcd usbcore
    NIP: c0000000000e1c00 LR: c0000000000034d4 CTR: 000000011012c440
    REGS: c00000003e2f3af0 TRAP: 0300   Not tainted  (2.6.37-rc6+)
    MSR: 9000000000001032 <ME,IR,DR>  CR: 48044444  XER: 20000000
    DAR: 00000001ffb9db50, DSISR: 0000000040000000
    TASK = c00000003e1a00a0[2088] 'emacs' THREAD: c00000003e2f0000 CPU: 1
    GPR00: 0000000000000001 c00000003e2f3d70 c00000000084e0d0 c0000000008816e8
    GPR04: 000000001034c678 000000001032e8f9 0000000010336540 0000000040020000
    GPR08: 0000000040020000 00000001ffb9db40 c00000003e2f3e30 0000000060000000
    GPR12: 100000000000f032 c00000000fff0280 000000001032e8c9 0000000000000008
    GPR16: 00000000105be9c0 00000000105be950 00000000105be9b0 00000000105be950
    GPR20: 00000000ffb9dc50 00000000ffb9dbf0 00000000102f0000 00000000102f0000
    GPR24: 00000000102e0000 00000000102f0000 0000000010336540 c0000000009ded38
    GPR28: 00000000102e0000 c0000000000034d4 c0000000007ccb10 c00000003e2f3d70
    NIP [c0000000000e1c00] .trace_hardirqs_off+0xb0/0x1d0
    LR [c0000000000034d4] decrementer_common+0xd4/0x100
    Call Trace:
    [c00000003e2f3d70] [c00000003e2f3e30] 0xc00000003e2f3e30 (unreliable)
    [c00000003e2f3e30] [c0000000000034d4] decrementer_common+0xd4/0x100
    Instruction dump:
    81690000 7f8b0000 419e0018 f84a0028 60000000 60000000 60000000 e95f0000
    80030000 e92a0000 eb6301f8 2f800000 <eb890010> 41fe00dc a06d000a eb1e8050
    ---[ end trace 4ec7fd2be9240928 ]---
Reported-by: Joerg Sommer <joerg@alea.gnuu.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>