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    SubjectRe: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v5)
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    On Thu, 2010-01-14 at 11:26 -0500, Mathieu Desnoyers wrote:

    > It's this scenario that is causing problem. Let's consider this
    > execution:
    >
    > CPU 0 (membarrier) CPU 1 (another mm -> our mm)
    > <kernel-space> <kernel-space>
    > switch_mm()
    > smp_mb()
    > clear_mm_cpumask()
    > set_mm_cpumask()
    > smp_mb() (by load_cr3() on x86)
    > switch_to()
    > mm_cpumask includes CPU 1
    > rcu_read_lock()
    > if (CPU 1 mm != our mm)
    > skip CPU 1.
    > rcu_read_unlock()
    > current = next (1)
    > <switch back to user-space>
    > read-lock()
    > read gp, store local gp
    > barrier()
    > access critical section (2)
    >
    > So if we don't have any memory barrier between (1) and (2), the memory
    > operations can be reordered in such a way that CPU 0 will not send IPI
    > to a CPU that would need to have it's barrier() promoted into a
    > smp_mb().

    I'm still not getting it, sure we don't send an IPI, but it will have
    done an mb() in switch_mm() to become our mm, so even without the IPI it
    will have executed that mb we were after.





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