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

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Date:	Thu, 7 Jan 2010 19:30:10 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>, akpm@...ux-foundation.org,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com, laijs@...fujitsu.com,
	dipankar@...ibm.com
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
	barrier

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.

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