Re: [PATCH 08/12] x86/dumpstack: Pin the target stack in save_stack_trace_tsk()

From: Josh Poimboeuf
Date: Thu Sep 15 2016 - 15:19:51 EST

On Thu, Sep 15, 2016 at 11:41:25AM -0700, Andy Lutomirski wrote:
> I also wouldn't mind trying to do something to prevent ever dumping
> the stack of an actively running task. It's definitely safe to dump:
> - current
> - any task that's stopped via ptrace, etc
> - any task on the current CPU if running atomically enough that the
> task can't migrate (which probably covers the nasty NMI cases, I hope)
> What's *not* safe AFAIK is /proc/PID/stack. I don't know if we can
> somehow fix that short of actually sending an interrupt or NMI to
> freeze the task if it's running. I'm also not sure it's worth
> worrying about it.

Yeah, I proposed a fix for /proc/PID/stack a while back:

My idea was to use task_rq_lock() to lock the runqueue and then check
tsk->on_cpu. I think Peter wasn't too keen on it.

We'll need something similar for the livepatch consistency model.