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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>,
       Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@elte.hu>,
       akpm@linux-foundation.org, josh@joshtriplett.org, tglx@linutronix.de,
       Valdis.Kletnieks@vt.edu, dhowells@redhat.com, laijs@cn.fujitsu.com,
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Date: Thu, 14 Jan 2010 13:52:12 -0500
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On Thu, 2010-01-14 at 13:37 -0500, Mathieu Desnoyers wrote:

> To make this painfully clear, I'll reorder the accesses to match that of
> the CPU to memory:
> 
>        CPU 0 (membarrier)                  CPU 1 (another mm -our mm)
>        <user-space>
>                                            <kernel-space>
>                                            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>
>                                            urcu read lock()
>                                              access critical section data (3)
>        memory access before membarrier
>        <call sys_membarrier()>
>        smp_mb()
>        mm_cpumask includes CPU 1
>        rcu_read_lock()
>        if (CPU 1 mm != our mm)
>          skip CPU 1.

I still don't see how the above conditional fails?

-- Steve

>        rcu_read_unlock()
>        smp_mb()
>        <return to user-space>
>        memory access after membarrier
>                                              current = next (1) (buffer flush)
>                                              read gp
>                                              store local gp (2)
> 
> This should make the problem a bit more evident. Access (3) is done
> outside of the read-side C.S. as far as the userspace synchronize_rcu()
> is concerned.
> 
> Thanks,
> 
> Mathieu
> 
> 


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