Re: [RFC PATCH 6/9] livepatch: create per-task consistency model

[Vojtech Pavlik <vojtech@suse.com>]

On Thu, Feb 19, 2015 at 10:52:51AM +0100, Peter Zijlstra wrote:

> On Wed, Feb 18, 2015 at 09:44:44PM +0100, Vojtech Pavlik wrote:
> > For live patching it doesn't matter whether code is running, sleeping or
> > frozen.
> > 
> > What matters is whether there is state before patching that may not be
> > valid after patching.
> > 
> > For userspace tasks, the exit from a syscall is a perfect moment for
> > switching to the "after" state, as all stacks, and thus all local
> > variables are gone and no local state exists in the kernel for the
> > thread.
> > 
> > The freezer is a logical choice for kernel threads, however, given that
> > kernel threads have no defined entry/exit point and execute within a
> > single main function, local variables stay and thus local state persists
> > from before to after freezing.
> > 
> > Defining that no local state within a kernel thread may be relied upon
> > after exiting from the freezer is certainly possible, and is already
> > true for many kernel threads.
> > 
> > It isn't a given property of the freezer itself, though. And isn't
> > obvious for author of new kernel threads either.
> > 
> > The ideal solution would be to convert the majority of kernel threads to
> > workqueues, because then there is a defined entry/exit point over which
> > state isn't transferred. That is a lot of work, though, and has other
> > drawbacks, particularly in the realtime space.
> 
> kthread_park() functionality seems to be exactly what you want.

It might be exactly that, indeed. The requrement of not just cleaning
up, but also not using contents of local variables from before parking
would need to be documented.

And kernel threads would need to start using it, too. I have been able
to find one instance where this functionality is actually used. So it is
again a matter of a massive patch adding that, like with the approach of
converting kernel threads to workqueues.

By the way, if kthread_park() was implemented all through the kernel,
would we still need the freezer for kernel threads at all? Since parking
seems to be stronger than freezing, it could also be used for that
purpose.

Vojtech