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Date:   Fri, 14 Feb 2020 22:53:05 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     paulmck@kernel.org
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org,
        kernel-team@fb.com, mingo@kernel.org, jiangshanlai@gmail.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
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        tglx@linutronix.de, peterz@infradead.org, dhowells@redhat.com,
        edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com,
        joel@joelfernandes.org,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH tip/core/rcu 22/30] rcu: Don't flag non-starting GPs
 before GP kthread is running
Message-ID: <20200214225305.48550d6a@oasis.local.home>
In-Reply-To: <20200214235607.13749-22-paulmck@kernel.org>
References: <20200214235536.GA13364@paulmck-ThinkPad-P72>
        <20200214235607.13749-22-paulmck@kernel.org>
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On Fri, 14 Feb 2020 15:55:59 -0800
paulmck@kernel.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