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

Steven Rostedt
2010-01-14T21:26:59Z

On Thu, 2010-01-14 at 14:33 -0500, Mathieu Desnoyers wrote:

> It's a case where CPU 1 switches from our mm to another mm:
>
> CPU 0 (membarrier) CPU 1 (another mm -our mm)
> <user-space> <user-space>
> <buffered access C.S. data>
> urcu read unlock()
> barrier()
> store local gp
> <kernel-space>
> rq->curr = next (1)
> memory access before membarrier
> <call sys_membarrier()>
> smp_mb()
> mm_cpumask includes CPU 1
> rcu_read_lock()
> if (cpu_curr(1)->mm != our mm)
> skip CPU 1 -> here, rq->curr new version is already visible
> rcu_read_unlock()
> smp_mb()
> <return to user-space>
> memory access after membarrier
> -> this is where we allow freeing
> the old structure although the
> buffered access C.S. data is
> still in flight.

OK, yeah this makes much more sense.

-- Steve

> User-space access C.S. data (2)
> (buffer flush)
> 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>
> current = next (1) (buffer flush)
> access critical section data (3)
>
> As we can see, the reordering of (1) and (2) is problematic, as it lets
> the check skip over a CPU that have global side-effects not committed to
> memory yet.