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

Steven Rostedt
2010-01-14T18:52:16Z

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
>
>