add new notifier function ,take4

Hi,

changelog take3 -> take4

- Rebased 2.6.25-mm1
- Add a document
- Add kdump on panic_notifier

These patches add new notifier function and implement it to panic_notifier_list. We used the hardcoded notifier chain so far, but it was not flexible. New notifier is very flexible, because user can change a list of order by control files.

And, third patch moves crash_kexec() to panic_notifier. It helps us to do something before taking a crash dump. It's useful for some RAS tools developer. If you want to use it, you have to set config option DUMP_ON_PANIC_NOTIFIER to Y. Default value of DUMP_ON_PANIC_NOTIFIER is N.
If you set DUMP_ON_PANIC_NOTIFIER to N, kdump has no difference before.

------
Example)

# cd /sys/kernel/notifiers/
# ls
panic_notifier_list
# cd panic_notifier_list/
# ls
ipmi_msghandler  ipmi_wdog
# insmod notifier_test.ko
# ls
ipmi_msghandler  ipmi_wdog  notifier_test1  notifier_test2
# cat */priority
200
150
500
1000
Kernel panic - not syncing: Panic by panic_module.
__tunable_atomic_notifier_call_chain enter
notifier_test: notifier_test_panic2() is called.
notifier_test: notifier_test_panic() is called.
msg_handler:panic_event was called.
ipmi_wdog:wdog_panic_handler was called.

.....(reboot)

# cd /sys/kernel/notifiers/panic_notifier_list/
# ls
ipmi_msghandler  ipmi_wdog  notifier_test1  notifier_test2
# cat */priority
200
150
500
1000
# echo 10000 > ipmi_msghandler/priority
# echo 5000 > ipmi_wdog/priority
# echo 3000 > notifier_test1/priority
# echo 1500 > notifier_test2/priority
# cat */priority
10000
5000
3000
1500
Kernel panic - not syncing: Panic by panic_module.
__tunable_atomic_notifier_call_chain enter
msg_handler:panic_event was called.
ipmi_wdog:wdog_panic_handler was called.
notifier_test: notifier_test_panic() is called.
notifier_test: notifier_test_panic2() is called.

--
Takenori Nagano <[email protected]>

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