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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 12:54 -0500, Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz@infradead.org) wrote:
> > 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:
> > >
>
> (slightly augmented)
>
> 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()
> memory access before membarrier
> <call sys_membarrier()>
> smp_mb()
> mm_cpumask includes CPU 1
> rcu_read_lock()
> if (CPU 1 mm != our mm)

But here, CPU 1 updated its mm already and did a
smp_mb, won't that make us send the smp_mb
anyway?

-- Steve

> skip CPU 1.
> rcu_read_unlock()
> smp_mb()
> <return to user-space>
> current = next (1)
> <switch back to user-space>
> urcu read lock()
> read gp
> store local gp (2)
> barrier()
> access critical section data (3)
> memory access after membarrier




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