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

Oleg Nesterov
2010-01-07T18:31:28Z

On 01/07, Peter Zijlstra wrote:
>
> On Wed, 2010-01-06 at 23:40 -0500, Mathieu Desnoyers wrote:
>
> http://marc.info/?t=126283939400002
>
> > Index: linux-2.6-lttng/kernel/sched.c
> > ===================================================================
> > --- linux-2.6-lttng.orig/kernel/sched.c 2010-01-06 22:11:32.000000000 -0500
> > +++ linux-2.6-lttng/kernel/sched.c 2010-01-06 23:20:42.000000000 -0500
> > @@ -10822,6 +10822,36 @@ struct cgroup_subsys cpuacct_subsys = {
> > };
> > #endif /* CONFIG_CGROUP_CPUACCT */
> >
> > +/*
> > + * Execute a memory barrier on all CPUs on SMP systems.
> > + * Do not rely on implicit barriers in smp_call_function(), just in case they
> > + * are ever relaxed in the future.
> > + */
> > +static void membarrier_ipi(void *unused)
> > +{
> > + smp_mb();
> > +}
> > +
>
> Also, there was some talk a while ago about IPIs implying memory
> barriers. Which I of course forgot all details about,.. at least sending
> one implies a wmb and receiving one an rmb, but it could be stronger,
> Oleg?

IIRC, it was decided that IPIs must imply mb(), but I am not sure
this is true on any arch/

However, even if IPI didn't imply mb(), I don't understand why it
is needed... After the quick reading of the original changelog in
http://marc.info/?l=linux-kernel&m=126283923115068

Thread A Thread B
prev mem accesses prev mem accesses
sys_membarrier() barrier()
follow mem accesses follow mem accesses

sys_membarrier() should "insert" mb() on behalf of B "instead"
of barrier(), right? But, if we send IPI, B enters kernel mode
and returns to user-mode. Should this imply mb() in any case?

Oleg.