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

Mathieu Desnoyers
2010-01-07T17:44:40Z

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> On Thu, Jan 07, 2010 at 06:18:36PM +0100, Peter Zijlstra wrote:
> > On Thu, 2010-01-07 at 08:52 -0800, Paul E. McKenney wrote:
> > > 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?
> >
> > Right, so I was thinking that since you want a mb to be executed when
> > calling sys_membarrier(). If you observe a matching ->mm but the cpu has
> > since scheduled, we're good since it scheduled (but we'll still send the
> > IPI anyway), if we do not observe it because the task gets scheduled in
> > after we do the iteration we're still good because it scheduled.
>
> Something like the following for sys_membarrier(), then?
>
> smp_mb();

This smp_mb() is redundant, as we issue it through the for_each_cpu loop
on the local CPU already.

> for_each_cpu(cpu, current->mm->cpu_vm_mask) {
> if (cpu_curr(cpu)->mm == current->mm)
> smp_call_function_single(cpu, func, NULL, 1);
> }
>
> Then the code changing ->mm on the other CPU also needs to have a
> full smp_mb() somewhere after the change to ->mm, but before starting
> user-space execution. Which it might well just due to overhead, but
> we need to make sure that someone doesn't optimize us out of existence.

I believe we also need one between execution of the userspace task and
change to ->mm. If we have these guarantees I think we are fine.

Mathieu

>
> Thanx, Paul
>
> > As to needing to keep rcu_read_lock() around the iteration, for sure we
> > need that to ensure the remote task_struct reference we take is valid.
> >

--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68