Re: [PATCH 2/4] kernel.h: Add non_block_start/end()

From: Peter Zijlstra
Date: Mon Dec 10 2018 - 09:47:24 EST

On Mon, Dec 10, 2018 at 03:13:37PM +0100, Michal Hocko wrote:
> I do not see any scheduler guys Cced and it would be really great to get
> their opinion here.
> On Mon 10-12-18 11:36:39, Daniel Vetter wrote:
> > In some special cases we must not block, but there's not a
> > spinlock, preempt-off, irqs-off or similar critical section already
> > that arms the might_sleep() debug checks. Add a non_block_start/end()
> > pair to annotate these.
> >
> > This will be used in the oom paths of mmu-notifiers, where blocking is
> > not allowed to make sure there's forward progress.
> Considering the only alternative would be to abuse
> preempt_{disable,enable}, and that really has a different semantic, I
> think this makes some sense. The cotext is preemptible but we do not
> want notifier to sleep on any locks, WQ etc.

I'm confused... what is this supposed to do?

And what does 'block' mean here? Without preempt_disable/IRQ-off we're
subject to regular preemption and execution can stall for arbitrary
amounts of time.

The Changelog doesn't yield any clues.