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

Ingo Molnar
2015-02-21T18:18:59Z

* Vojtech Pavlik <vojtech@suse.com> wrote:

> On Fri, Feb 20, 2015 at 08:49:01PM +0100, Ingo Molnar wrote:
>
> > > ... 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.
>
> Can you suggest a way how they would interfere? The
> transition happens on entering or returning from a
> syscall, there is no influence between individual tasks.

Well, a 'new' task does not stop executing after returning
from the syscall, right? If it's stopped (until all
patching is totally complete) then you are right and I
concede your point.

If it's allowed to continue its workload then my point
stands: subsequent execution of 'new' tasks can interfere
with, slow down, interact with 'old' tasks trying to get
patched.

> > 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.
>
> The 'simple' method has to catch and freeze all tasks one
> by one in syscall entry/exit, at the kernel/userspace
> boundary, until all are frozen and then patch the system
> atomically.

Correct.

> This means that each and every sleeping task in the
> system has to be woken up in some way (sending a signal
> ...) to exit from a syscall it is sleeping in. Same for
> CPU hogs. All kernel threads need to be parked.

Yes - although I'd not use signals for this, signals have
side effects - but yes, something functionally equivalent.

> This is exactly what you need to do for kGraft to
> complete patching.

My understanding of kGraft is that by default you allow
tasks to continue 'in the new universe' after they are
patched. Has this changed or have I misunderstood the
concept?

Thanks,

Ingo