Re: live patching design (was: Re: [PATCH 1/3] sched: add sched_task_call())

From: Ingo Molnar
Date: Fri Feb 20 2015 - 14:49:14 EST

* Jiri Kosina <jkosina@xxxxxxx> wrote:

> > All fundamental pieces of the simple method are
> > necessary to get guaranteed time transition from the
> > complicated method: task tracking and transparent
> > catching of them, handling kthreads, etc.
> >
> > My argument is that the simple method should be
> > implemented first and foremost.
> >
> > Then people can do add-on features to possibly spread
> > out the new function versions in a more complicated way
> > if they want to avoid the stop-all-tasks transition -
> > although I'm not convinced about it: I'm sure sure many
> > sysadmins would like the bug patching to be over with
> > quickly and not have their systems in an intermediate
> > state like kgraft does it.
> ... the choice the sysadmins have here is either have the
> system running in an intermediate state, or have the
> system completely dead for the *same time*. Because to
> finish the transition successfully, all the tasks have to
> be woken up in any case.

That statement is false: an 'intermediate state' system
where 'new' tasks are still running is still running and
will interfere with the resolution of 'old' tasks.

> But I do get your point; what you are basically saying is
> that your preference is what kgraft is doing, and option
> to allow for a global synchronization point should be
> added in parallel to the gradual lazy migration.

I think you misunderstood: the 'simple' method I outlined
does not just 'synchronize', it actually executes the live
patching atomically, once all tasks are gathered and we
know they are _all_ in a safe state.

I.e. it's in essence the strong stop-all atomic patching
model of 'kpatch', combined with the reliable avoidance of
kernel stacks that 'kgraft' uses.

That should be the starting point, because it's the most
reliable method.


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