linux-kernel - Re: [PATCH 3/5] rcuwait: Introduce prepare_to and finish

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Date:   Thu, 23 Apr 2020 11:23:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     tglx@...utronix.de, pbonzini@...hat.com, bigeasy@...utronix.de,
        rostedt@...dmis.org, torvalds@...ux-foundation.org,
        will@...nel.org, joel@...lfernandes.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 3/5] rcuwait: Introduce prepare_to and finish_rcuwait

On Tue, Apr 21, 2020 at 09:07:37PM -0700, Davidlohr Bueso wrote:

> +static inline void prepare_to_rcuwait(struct rcuwait *w)
> +{
> +	rcu_assign_pointer(w->task, current);
> +}
> +
> +static inline void finish_rcuwait(struct rcuwait *w)
> +{
> +	WRITE_ONCE(w->task, NULL);

I think that wants to be:

	rcu_assign_pointer(w->task, NULL);

There is a special case in rcu_assign_pointer() that looses the barrier,
but it will keep the __rcu sparse people happy. That is w->task is
__rcu, and WRITE_ONCE ignores that etc.. blah.

The alternative is using RCU_INIT_POINTER() I suppose.

> +	__set_current_state(TASK_RUNNING);
> +}