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

[Peter Zijlstra]

On Tue, 2010-01-12 at 20:37 -0500, Mathieu Desnoyers wrote:
> +       for_each_cpu(cpu, tmpmask) {
> +               spin_lock_irq(&cpu_rq(cpu)->lock);
> +               mm = cpu_curr(cpu)->mm;
> +               spin_unlock_irq(&cpu_rq(cpu)->lock);
> +               if (current->mm != mm)
> +                       cpumask_clear_cpu(cpu, tmpmask);
> +       } 

Why not:

  rcu_read_lock();
  if (current->mm != cpu_curr(cpu)->mm)
    cpumask_clear_cpu(cpu, tmpmask);
  rcu_read_unlock();

the RCU read lock ensures the task_struct obtained remains valid, and it
avoids taking the rq->lock.

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