barriers: was: [RFC PATCH v2 17/18] livepatch: change to a per-task consistency model

[Petr Mladek]

On Wed 2016-05-04 12:02:36, Josh Poimboeuf wrote:
> On Wed, May 04, 2016 at 02:39:40PM +0200, Petr Mladek wrote:
> > On Thu 2016-04-28 15:44:48, Josh Poimboeuf wrote:
> > > Change livepatch to use a basic per-task consistency model.  This is the
> > > foundation which will eventually enable us to patch those ~10% of
> > > security patches which change function or data semantics.  This is the
> > > biggest remaining piece needed to make livepatch more generally useful.
> > 
> > I spent a lot of time with checking the memory barriers. It seems that
> > they are basically correct.  Let me use my own words to show how
> > I understand it. I hope that it will help others with review.
> 
> [...snip a ton of useful comments...]
> 
> Thanks, this will help a lot!  I'll try to incorporate your barrier
> comments into the code.

Thanks a lot.

> I also agree that kpatch_patch_task() is poorly named.  I was trying to
> make it clear to external callers that "hey, the task is getting patched
> now!", but it's internally inconsistent with livepatch code because we
> make a distinction between patching and unpatching.
> 
> Maybe I'll do:
> 
>   klp_update_task_patch_state()

I like it. It is long but it well describes the purpose.

Livepatch is using many state variables:

  + global:                klp_transition_patch, klp_target_state
  + per task specific:     TIF_PENDING_PATCH, patch_state
  + per each new function: transition, patched
  + per old function:      func_stack
  + per object:            patched, loaded
  + per patch:             enabled

The dependency between them and the workflow is important to
create a mental picture about the Livepatching. Good names
help with it.

Best Regards,
Petr