linux-kernel - Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier

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Date:   Tue, 3 Sep 2019 13:27:23 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Christoph Lameter <cl@...ux.com>,
        Kirill Tkhai <tkhai@...dex.ru>, Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC PATCH 1/2] Fix: sched/membarrier: p->mm->membarrier_state
 racy load

On Tue, Sep 3, 2019 at 1:11 PM Mathieu Desnoyers
<mathieu.desnoyers@...icios.com> wrote:
>
> +       cpus_read_lock();
> +       for_each_online_cpu(cpu) {

This would likely be better off using mm_cpumask(mm) instead of all
online CPU's.

Plus doing the rcu_read_lock() inside the loop seems pointless. Even
with a lot of cores, it's not going to loop _that_ many times for RCU
latency to be an issue.

              Linus