linux-kernel - Re: [PATCH 2/3] task: RCU protect tasks on the runqueue

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Date:   Tue, 3 Sep 2019 09:41:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.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>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH 2/3] task: RCU protect tasks on the runqueue

On Mon, Sep 02, 2019 at 11:52:01PM -0500, Eric W. Biederman wrote:

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2b037f195473..802958407369 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c

> @@ -3857,7 +3857,7 @@ static void __sched notrace __schedule(bool preempt)
>  
>  	if (likely(prev != next)) {
>  		rq->nr_switches++;
> -		rq->curr = next;
> +		rcu_assign_pointer(rq->curr, next);
>  		/*
>  		 * The membarrier system call requires each architecture
>  		 * to have a full memory barrier after updating

This one is sad; it puts a (potentially) expensive barrier in here. And
I'm not sure I can explain the need for it. That is, we've not changed
@next before this and don't need to 'publish' it as such.

Can we use RCU_INIT_POINTER() or simply WRITE_ONCE(), here?