task switch hook - crashes

[Krisztian Flautner <manowar@engin.umich.edu>]

I am experimenting with adding a few hooks to the kernel.
One of the hooks gets invoked every time the kernel switches
two tasks. To do this I added a single line to schedule() right
before the tasks are switched:

      ...
      if (task_switch_hook) task_switch_hook(prev, next)

      switch_to(prev, next, prev);
      ...

I then wrote a module that attaches a function to this hook
in its init_module() function. The problem is that if I do this,
the kernel crashes. Sometimes, the hooks are invoked a few times,
other times, the machine locks up immediately. I previous version
that compiled everything into the kernel worked fine.

I don't quite understand why this does not work. Is this due
to some virtual memory interactions? Maybe the problem is that
modules are vmalloc-ed and the hook function should be kmalloc-ed?

Any insights?


Thanks, -- Kris