Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v5)

Peter Zijlstra
2010-01-19T19:07:42Z

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?