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Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
 barrier (v5)
From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@elte.hu>,
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Date: Tue, 19 Jan 2010 20:06:46 +0100
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On Tue, 2010-01-19 at 19:37 +0100, Peter Zijlstra wrote:
> On Thu, 2010-01-14 at 14:33 -0500, Mathieu Desnoyers wrote:
> > It's a case where CPU 1 switches from our mm to another mm:
> > 
> >        CPU 0 (membarrier)                  CPU 1 (another mm -our mm)
> >        <user-space>                        <user-space>
> >                                            <buffered access C.S. data>
> >                                            urcu read unlock()
> >                                              barrier()
> >                                              store local gp
> >                                            <kernel-space>
> 
> OK, so the question is how we end up here, if its though interrupt
> preemption I think the interrupt delivery will imply an mb,

I keep thinking that, but I think we actually refuted that in an earlier
discussion on this patch.

>  if its a
> blocking syscall, the set_task_state() mb [*] should be there.
> 
> Then we also do:
> 
> 					clear_tsk_need_resched()
> 
> which is an atomic bitop (although does not imply a full barrier
> per-se).
> 
> >                                            rq->curr = next (1)

We could possibly look at placing that assignment in context_switch()
between switch_mm() and switch_to(), which should provide a mb before
and after I think, Ingo?

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