From: Arnd Bergmann <arnd@arndb.de>

Christian Bornträger noticed that the kernel can crash after <SysRq>-T.  It
appears that the show_task function gets called for all tasks, which does
not work if one of the tasks is running in a system call on another CPU.
In that case the result of thread_saved_pc and show_stack is undefined and
likely to cause a crash.

For tasks running in user space on other CPUs, show_task() is probably
harmless, but I'm not sure if that's true on all architectures.

The patch below is still racy for tasks that are about to sleep, but it
demonstrates the problem.