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Date: Wed, 7 Oct 2015 15:24:54 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
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        edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com,
        oleg@redhat.com, bobby.prani@gmail.com
Subject: Re: [PATCH tip/core/rcu 04/18] rcu: Use single-stage IPI algorithm
 for RCU expedited grace period
Message-ID: <20151007132454.GA3604@twins.programming.kicks-ass.net>
References: <20151006162907.GA12020@linux.vnet.ibm.com>
 <1444148977-14108-1-git-send-email-paulmck@linux.vnet.ibm.com>
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On Tue, Oct 06, 2015 at 09:29:23AM -0700, Paul E. McKenney wrote:
> @@ -3494,19 +3483,21 @@ static int sync_rcu_preempt_exp_done(struct rcu_node *rnp)
>   * recursively up the tree.  (Calm down, calm down, we do the recursion
>   * iteratively!)
>   *
> - * Caller must hold the root rcu_node's exp_funnel_mutex.
> + * Caller must hold the root rcu_node's exp_funnel_mutex and the
> + * specified rcu_node structure's ->lock.
>   */
> -static void __maybe_unused rcu_report_exp_rnp(struct rcu_state *rsp,
> -					      struct rcu_node *rnp, bool wake)
> +static void __rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
> +				 bool wake, unsigned long flags)
> +	__releases(rnp->lock)
>  {
> -	unsigned long flags;
>  	unsigned long mask;
>  
> -	raw_spin_lock_irqsave(&rnp->lock, flags);
> -	smp_mb__after_unlock_lock();

	lockdep_assert_held(&rnp->lock);

> +/*
> + * Report expedited quiescent state for specified node.  This is a
> + * lock-acquisition wrapper function for __rcu_report_exp_rnp().
> + *
> + * Caller must hold the root rcu_node's exp_funnel_mutex.
> + */
> +static void __maybe_unused rcu_report_exp_rnp(struct rcu_state *rsp,
> +					      struct rcu_node *rnp, bool wake)
> +{
> +	unsigned long flags;

	lockdep_assert_held(&rcu_get_root(rsp)->exp_funnel_mutex);

> +
> +	raw_spin_lock_irqsave(&rnp->lock, flags);
> +	smp_mb__after_unlock_lock();
> +	__rcu_report_exp_rnp(rsp, rnp, wake, flags);
> +}


Etc.. these are much harder to ignore than comments.