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

Paul E. McKenney
2010-01-07T17:55:40Z

On Thu, Jan 07, 2010 at 12:44:35PM -0500, Mathieu Desnoyers wrote:
> * 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.

But we need to do the smp_mb() -before- checking the first cpu_curr(cpu)->mm.

> > 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.

Agreed, in case an outgoing RCU read-side critical section does a store
into an RCU-protected data structure. Unconventional, but definitely
permitted.

Thanx, Paul

> 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
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