linux-kernel - Re: [PATCH tip/core/rcu 22/30] rcu: Don't flag non-starting GPs before GP kthread is running

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Message-ID: <20200214225305.48550d6a@oasis.local.home>
Date:   Fri, 14 Feb 2020 22:53:05 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     paulmck@...nel.org
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        dipankar@...ibm.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, dhowells@...hat.com,
        edumazet@...gle.com, fweisbec@...il.com, oleg@...hat.com,
        joel@...lfernandes.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH tip/core/rcu 22/30] rcu: Don't flag non-starting GPs
 before GP kthread is running

On Fri, 14 Feb 2020 15:55:59 -0800
paulmck@...nel.org wrote:

> @@ -1252,10 +1252,10 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp)
>   */
>  static void rcu_gp_kthread_wake(void)
>  {
> -	if ((current == rcu_state.gp_kthread &&
> +	if ((current == READ_ONCE(rcu_state.gp_kthread) &&
>  	     !in_irq() && !in_serving_softirq()) ||
>  	    !READ_ONCE(rcu_state.gp_flags) ||
> -	    !rcu_state.gp_kthread)
> +	    !READ_ONCE(rcu_state.gp_kthread))
>  		return;

This looks buggy. You have two instances of
READ_ONCE(rcu_state.gp_thread), which means they can be different. Is
that intentional?

-- Steve