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Date: Wed, 2 Oct 2013 17:49:01 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>, Mel Gorman <mgorman@suse.de>,
        Rik van Riel <riel@redhat.com>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Ingo Molnar <mingo@kernel.org>, Andrea Arcangeli <aarcange@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/3] rcu: Create rcu_sync infrastructure
Message-ID: <20131002154901.GA13389@redhat.com>
References: <20131002145655.361606532@infradead.org> <20131002150518.675931976@infradead.org>
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On 10/02, Peter Zijlstra wrote:
>
> From: Oleg Nesterov <oleg@redhat.com>

Thanks! I was writing the patch, and I chose almost the same naming ;)

> Signed-off-by: Peter Zijlstra <peterz@infradead.org>

Signed-off-by: Oleg Nesterov <oleg@redhat.com>

In fact I'd like to add my sob to 1/3 and 3/3 as well.


Paul, to remind, this is only the first step. I am going to send
the following improvements:

	1. Add rcu_sync->exlusive. The change is simple, just we
	   need s/wait_queue_head_t/completion/ in rcu_sync_struct
	   and a couple of "if (rss->exclusive)" checks in enter/exit.

	2. rcu_sync_enter() should return !!need_sync. This can help
	   in exclusive mode.

	3. rcu_sync_struct needs more function pointers (perhaps we
	   should add a single rcu_sync_struct->ops pointer but this
	   is minor). See below.

But let me repeat just in case, we should do this later.
And once this series is applied, I'll change percpu_rw_semaphore.


> +struct rcu_sync_struct {
> +	int			gp_state;
> +	int			gp_count;
> +	wait_queue_head_t	gp_wait;
> +
> +	int			cb_state;
> +	struct rcu_head		cb_head;
> +
> +	void (*sync)(void);
> +	void (*call)(struct rcu_head *, void (*)(struct rcu_head *));

Yes, and we also need rcu_sync_struct->barrier(). From the patch I was
working on:

	void rcu_sync_wait_for_callback(struct rcu_sync *sync)
	{
		int cb_state;

		BUG_ON(sync->gp_count);

		spin_lock_irq(&sync->state_lock);
		if (sync->cb_state == CB_REPLAY)
			sync->cb_state = CB_PENDING;
		cb_state = sync->cb_state;
		spin_unlock_irq(&sync->state_lock);

		if (cb_state != CB_IDLE) {
			rcu_barrier_sched();
			BUG_ON(sync->cb_state != CB_IDLE);
		}
	}

It should be called if you are going to kfree the object.

Perhaps another rcu_sync_struct->state_change(new_state) callback (set
by the user) makes sense too, this can help (for example) to implement
the array of semaphores with a single rcu_sync_struct (freeze_super).

Thanks.

Oleg.