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

Peter Zijlstra
2010-01-14T17:04:17Z

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.