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

[Peter Zijlstra <peterz@infradead.org>]

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.