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

[Eric W. Biederman]

Michael Holzheu <holzheu at linux.vnet.ibm.com> writes:

> 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?

smp_send_stop is insufficiently reliable to be used before crash_kexec.

My first reaction would be to test oops_in_progress and wait until
oops_in_progress == 1 before calling smp_send_stop.

Eric