Handle the NT_PRSTATUS lost for the "bt" command
 

The purpose of this patch is to work out "bt" command for the diskdump
which NT_PRSTATUS note could not be saved by IPI lost.
I think IPI is possibly lost by panic under the serious crashed condition.

I noticed that "bt" failed in my ppc environment
when the NT_PRSTATUS notes are lost on some CPUs while IPI delivery.
Then, I made CPU map for prstatus in diskdump more correctable
by checking a validation of crash_notes field.

I've tested this problem by patching kernel like,
- kernel/kexec.c
void crash_save_cpu(struct pt_regs *regs, int cpu)
{
+ if (current->pid == 0)
+ /* this cpu was idle; nothing to capture */
+ return;

It looks terrible and impractical test case but actually
I met this code in my using distro's kernel.
I couldn't reproduce actual IPI lost case, then fortunately, use this
as a example of the causes if IPI could not be delivered to other CPUs.

=> Taking diskdump by sysrq+c and makedumpfile.

crash> help -D | grep notes
num_prstatus_notes: 1
notes_buf: 10ba91a8
notes[0]: 10ba91a8
crash> help -k | grep cpus
cpus: 8
cpus_override: (null)
crash> bt
PID: 1001 TASK: ea62b000 CPU: 2 COMMAND: "bash"
Segmentation fault

Since seven idle cpus did not save NT_PRSTATUS note,
crash could not handle CPU#2's note where is located as CPU#0's.

With this patch, crash get to work out with correct CPU map to prstatus.

WARNING: catch lost crash_notes at cpu#0
WARNING: catch lost crash_notes at cpu#1
WARNING: catch lost crash_notes at cpu#3
WARNING: catch lost crash_notes at cpu#4
WARNING: catch lost crash_notes at cpu#5
WARNING: catch lost crash_notes at cpu#6
WARNING: catch lost crash_notes at cpu#7
crash.fix> help -D | grep notes
num_prstatus_notes: 1
notes_buf: 107a3378
notes[2]: 107a3378
crash.fix> help -k | grep cpus
cpus: 8
cpus_override: (null)
crash.fix> bt
PID: 1001 TASK: ea62b000 CPU: 2 COMMAND: "bash"

R0: 00000001 R1: eb793e60 R2: ea62b000 R3: 00000063
R4: 00000000 R5: ffffffff R6: c043ba2c R7: 00000000
R8: 00008000 R9: 00000000 R10: 00000000 R11: eb793e70
R12: 28242444 R13: 100b8448 R14: 100b07b8 R15: 100b0894
R16: 00000000 R17: 00000000 R18: 00000000 R19: 1006d270
R20: 00000000 R21: 100f0430 R22: 00000000 R23: 00000001
R24: c08f1ac8 R25: 00029002 R26: c08f1bac R27: c08d0000
R28: 00000000 R29: c09ada48 R30: 00000063 R31: eb793e60
NIP: c0423378 MSR: 00021002 OR3: c09ada48 CTR: c0423344
LR: c0423d8c XER: 00000000 CCR: 28242444 MQ: 00008000
DAR: 00000000 DSISR: 00800000 Syscall Result: eb793e60
NIP [00000000c0423378] sysrq_handle_crash
LR [00000000c0423d8c] __handle_sysrq

#0 [eb793e60] sysrq_handle_crash at c0423378
: snip

Thanks,
Toshi

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