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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 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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