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Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
 barrier (v5)
From: Steven Rostedt <rostedt@goodmis.org>
Reply-To: rostedt@goodmis.org
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
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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>
>                                            rq->curr = next (1)
>        memory access before membarrier
>        <call sys_membarrier()>
>        smp_mb()
>        mm_cpumask includes CPU 1
>        rcu_read_lock()
>        if (cpu_curr(1)->mm != our mm)
>          skip CPU 1     -> here, rq->curr new version is already visible
>        rcu_read_unlock()
>        smp_mb()
>        <return to user-space>
>        memory access after membarrier
>        -> this is where we allow freeing
>           the old structure although the
>           buffered access C.S. data is
>           still in flight.

OK, yeah this makes much more sense.

-- Steve

>                                            User-space access C.S. data (2)
>                                              (buffer flush)
>                                            switch_mm()
>                                              smp_mb()
>                                              clear_mm_cpumask()
>                                              set_mm_cpumask()
>                                              smp_mb() (by load_cr3() on x86)
>                                            switch_to()
>                                              <buffered current = next>
>                                            <switch back to user-space>
>                                              current = next (1) (buffer flush)
>                                            access critical section data (3)
> 
> As we can see, the reordering of (1) and (2) is problematic, as it lets
> the check skip over a CPU that have global side-effects not committed to
> memory yet.


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