Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 25, 2017 at 04:59:36PM -0700, Paul E. McKenney wrote:
> On Tue, Jul 25, 2017 at 11:55:10PM +0200, Peter Zijlstra wrote:

> > People always do crazy stuff, but what surprised me is that such s patch
> > got merged in urcu even though its known broken for a number of
> > architectures.
> 
> It did not get merged into urcu.  It is instead used directly by a
> number of people for a number of concurrent algorithms.

Yah, Mathieu also already pointed that out. It seems I really cannot
deal with github well -- that website always terminally confuses me.

> > > But it would not be hard for userspace code to force IPIs by repeatedly
> > > awakening higher-priority threads that sleep immediately after being
> > > awakened, right?
> > 
> > RT tasks are not readily available to !root, and the user might have
> > been constrained to a subset of available CPUs.
> 
> So non-idle non-nohz CPUs never get IPIed for wakeups of SCHED_OTHER
> threads?

Sure, but SCHED_OTHER auto throttles in that if there's anything else to
run, you get to wait. So you can't generate an IPI storm with it. Also,
again, we can be limited to a subset of CPUs.

> > My thinking was that if we observe '!= mm' that CPU will have to do a
> > context switch in order to make it true. That context switch will
> > provide the ordering we're after so all is well.
> > 
> > Quite possible there's a hole in, but since I'm running on fumes someone
> > needs to spell it out for me :-)
> 
> This would be the https://marc.info/?l=linux-kernel&m=126349766324224&w=2
> URL below.
> 
> Which might or might not still be applicable.

I think we actually have those two smp_mb()'s around the rq->curr
assignment.

we have smp_mb__before_spinlock(), which per the argument here:

  https://lkml.kernel.org/r/20170607162013.755917928@infradead.org

is actually a full MB, irrespective of that weird smp_wmb() definition
we have now. And we have switch_mm() on the other side.

> > > I was intending to base this on the last few versions of a 2010 patch,
> > > but maybe things have changed:
> > > 
> > > 	https://marc.info/?l=linux-kernel&m=126358017229620&w=2
> > > 	https://marc.info/?l=linux-kernel&m=126436996014016&w=2
> > > 	https://marc.info/?l=linux-kernel&m=126601479802978&w=2
> > > 	https://marc.info/?l=linux-kernel&m=126970692903302&w=2
> > > 
> > > Discussion here:
> > > 
> > > 	https://marc.info/?l=linux-kernel&m=126349766324224&w=2
> > > 
> > > The discussion led to acquiring the runqueue locks, as there was
> > > otherwise a need to add code to the scheduler fastpaths.
> > 
> > TL;DR..  that's far too much to trawl through.
> 
> So we re-derive it from first principles instead?  ;-)

Yep, that's what I usually do anyway, who knows what kind of crazy our
younger selves were up to ;-)