Re: [PATCH 2/3] rcu: Create rcu_sync infrastructure

[Oleg Nesterov <oleg@redhat.com>]

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.