kdump: crash_kexec()-smp_send_stop() race in panic

[Michael Holzheu]

Hello Vivek,

In our tests we ran into the following scenario:

Two CPUs have called panic at the same time. The first CPU called
crash_kexec() and the second CPU called smp_send_stop() in panic()
before crash_kexec() finished on the first CPU. So the second CPU
stopped the first CPU and therefore kdump failed.

1st CPU:
panic()->crash_kexec()->mutex_trylock(&kexec_mutex)-> do kdump

2nd CPU:
panic()->crash_kexec()->kexec_mutex already held by 1st CPU
       ->smp_send_stop()-> stop CPU 1 (stop kdump)

How should we fix this problem? One possibility could be to do
smp_send_stop() before we call crash_kexec().

What do you think?

Michael