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

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Date:	Wed, 13 Jan 2010 17:38:58 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org, josh@...htriplett.org,
	tglx@...utronix.de, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	laijs@...fujitsu.com, dipankar@...ibm.com
Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
 barrier (v5)

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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