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

[Paul E. McKenney]

On Thu, Jul 27, 2017 at 12:14:26PM +0200, Peter Zijlstra wrote:
> On Tue, Jul 25, 2017 at 12:36:12PM -0700, Paul E. McKenney wrote:
> > This horse is already out, so trying to shut the gate won't be effective.
> 
> So I'm not convinced it is. The mprotect() hack isn't portable as we've
> established and on x86 where it does work, it doesn't (much) perturb
> tasks not related to our process because we keep a tight mm_cpumask().

Wrong.  People are using it today, portable or not.  If we want them
to stop using it, we need to give them an alternative.  Period.

> And if there are other (unpriv.) means of spraying IPIs around, we
> should most certainly look at fixing those, not just shrug and make
> matters worse.

We need to keep an open mind for a bit.  If there was a trivial
solution, we would have implemented it back in 2010.

							Thanx, Paul