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

[Peter Zijlstra]

On Thu, Jul 27, 2017 at 06:08:16AM -0700, Paul E. McKenney wrote:

> > So I think we need either switch_mm() or switch_to() to imply a full
> > barrier for this to work, otherwise we get:
> > 
> >   CPU0				CPU1
> > 
> > 
> >   lock rq->lock
> >   mb
> > 
> >   rq->curr = A
> > 
> >   unlock rq->lock
> > 
> >   lock rq->lock
> >   mb
> > 
> > 				sys_membarrier()
> > 
> > 				mb
> > 
> > 				for_each_online_cpu()
> > 				  p = A
> > 				  // no match no IPI
> > 
> > 				mb
> >   rq->curr = B
> > 
> >   unlock rq->lock
> > 
> > 
> > And that's bad, because now CPU0 doesn't have an MB happening _after_
> > sys_membarrier() if B matches.
> 
> Yes, this looks somewhat similar to the scenario that Mathieu pointed out
> back in 2010: https://marc.info/?l=linux-kernel&m=126349766324224&w=2

Yes. Minus the mm_cpumask() worries.

> > So without audit, I only know of PPC and Alpha not having a barrier in
> > either switch_*().
> > 
> > x86 obviously has barriers all over the place, arm has a super duper
> > heavy barrier in switch_to().
> 
> Agreed, if we are going to rely on ->mm, we need ordering on assignment
> to it.

Right, Boqun provided this reordering to show the problem:

  CPU0                                CPU1
 
 
  <in process X>
  lock rq->lock
  mb
 
  rq->curr = A
 
  unlock rq->lock
 
  <switch to process A>
 
  lock rq->lock
  mb
  read Y(reordered)<---+
                       |        store to Y
                       |
                       |        sys_membarrier()
                       |
                       |        mb
                       |
                       |        for_each_online_cpu()
                       |          p = A
                       |          // no match no IPI
                       |
                       |        mb
                       |
                       |        store to X
  rq->curr = B         |
                       |
  unlock rq->lock      |
  <switch to B>        |
  read X               |
                       |
  read Y --------------+