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Date:   Thu, 23 Apr 2020 11:23:14 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Davidlohr Bueso <dave@stgolabs.net>
Cc:     tglx@linutronix.de, pbonzini@redhat.com, bigeasy@linutronix.de,
        rostedt@goodmis.org, torvalds@linux-foundation.org,
        will@kernel.org, joel@joelfernandes.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Davidlohr Bueso <dbueso@suse.de>
Subject: Re: [PATCH 3/5] rcuwait: Introduce prepare_to and finish_rcuwait
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References: <20200422040739.18601-1-dave@stgolabs.net>
 <20200422040739.18601-4-dave@stgolabs.net>
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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);
> +}