Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Jan 07, 2010 at 09:44:15AM +0100, Peter Zijlstra wrote:
> On Wed, 2010-01-06 at 22:35 -0800, Josh Triplett wrote:
> > 
> > The number of threads doesn't matter nearly as much as the number of
> > threads typically running at a time compared to the number of
> > processors.  Of course, we can't measure that as easily, but I don't
> > know that your proposed heuristic would approximate it well.
> 
> Quite agreed, and not disturbing RT tasks is even more important.

OK, so I stand un-Reviewed-by twice in one morning.  ;-)

> A simple:
> 
>   for_each_cpu(cpu, current->mm->cpu_vm_mask) {
>      if (cpu_curr(cpu)->mm == current->mm)
>         smp_call_function_single(cpu, func, NULL, 1);
>   }
> 
> seems far preferable over anything else, if you really want you can use
> a cpumask to copy cpu_vm_mask in and unset bits and use the mask with
> smp_call_function_any(), but that includes having to allocate the
> cpumask, which might or might not be too expensive for Mathieu.

This would be vulnerable to the sys_membarrier() CPU seeing an old value
of cpu_curr(cpu)->mm, and that other task seeing the old value of the
pointer we are trying to RCU-destroy, right?

							Thanx, Paul