Re: [Qemu-devel] [PATCH 14/23] userfaultfd: wake pending userfaults

[Peter Zijlstra]

On Thu, Oct 22, 2015 at 03:20:15PM +0200, Andrea Arcangeli wrote:

> If schedule spontaneously wakes up a task in TASK_KILLABLE state that
> would be a bug in the scheduler in my view. Luckily there doesn't seem
> to be such a bug, or at least we never experienced it.

Well, there will be a wakeup, just not the one you were hoping for.

We have code that does:

        @cond = true;
        get_task_struct(p);
        queue(p)

                                /* random wait somewhere */
                                for (;;) {
                                        prepare_to_wait();
                                        if (@cond)
                                          break;

                                ...

                                handle_userfault()
                                  ...
                                  schedule();
        ...

        dequeue(p)
        wake_up_process(p) ---> wakeup without userfault wakeup


These races are (extremely) rare, but they do exist. Therefore one must
never assume schedule() will not spuriously wake because of these
things.

Also, see:

lkml.kernel.org/r/address@hidden