linux-kernel - Re: [PATCH] rcu_sync: simplify the state machine, introduce __rcu_sync

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Date:	Mon, 18 Jul 2016 15:44:20 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, tj@...nel.org,
	john.stultz@...aro.org, dimitrysh@...gle.com, romlem@...gle.com,
	ccross@...gle.com, tkjos@...gle.com
Subject: Re: [PATCH] rcu_sync: simplify the state machine, introduce
	__rcu_sync_enter()

On 07/18, Peter Zijlstra wrote:
>
> I think I ended up with this:
>
>  .---->	GP_IDLE <--------------.
>  |	  |                    |
>  |	  | __rcu_sync_enter() | <GP> / rcu_sync_exit()
>  |	  v                    |
>  |	GP_ENTER --------------'
>  |	  |
>  |  <GP>  |
>  |        v
>  |	GP_PASSED <---------.
>  |	  |                 |
>  |	  | rcu_sync_exit() | <GP> / __rcu_sync_enter()
>  |	  v                 |
>  `----- GP_EXIT ------------'
> 	  ^
>     <GP>  | __rcu_sync_enter() + rcu_sync_exit()
> 	  v
> 	GP_RETRY

Thanks! I'll include this into the changelog.

> > static void rcu_sync_call(struct rcu_sync *rsp)
> > {
> > 	// TODO: THIS IS SUBOPTIMAL. We want to call it directly
> > 	// if rcu_blocking_is_gp() == T, but it has might_sleep().
>
> Not sure I get that comment..

I meant, we actually want

	static void rcu_sync_call(struct rcu_sync *rsp)
	{
		if (rcu_blocking_is_gp())
			rcu_sync_func(rsp);
		else
			gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func)
	}

but this needs some other simple changes. rcu_sync_func() needs
the same spinlock and rcu_blocking_is_gp() calls might_sleep().

We can simply move rcu_sync_call() outside of rsp->rss_lock, but
then we will need more comments to explain why we can't race with
enter/exit from someone else. Or introduce __rcu_blocking_is_gp()
without might_sleep(), but this needs a trivial change outside of
rcu/sync.c.

We will see. This is simple anyway.

> > 	spin_lock_irqsave(&rsp->rss_lock, flags);
> > 	if (rsp->gp_count) {
> > 		/*
> > 		 * We're at least a GP after the first __rcu_sync_enter().
> > 		 */
> > 		rsp->gp_state = GP_PASSED;
>
> So we can end up here in two ways afaict.
>
> The simple way: someone called __rcu_sync_enter(), we go IDLE -> ENTER
> with a raised count. Once the GP passes, we get here, observe the raised
> count and advance to PASSED.
>
> The more involved way: we were EXIT and someone calls __rcu_sync_enter()
> to raise the count again. The callback from rcu_sync_exit() was still
> pending and once we get here we observe the raised count and voila.

Yes, yes.

> Now, since state != IDLE, I suppose this is valid, but it does hurt my
> brain.

Simply put, if rsp->gp_count != 0 we do not care about the history and
GP_PASSED is always correct when rcu callback is called, this obviously
means that we passed a GP.

Except GP_IDLE -> GP_PASSED transition is wrong, but this must not be
possible because only rcu callback can set GP_IDLE and only if !gp_count,
so we must always have at least one GP in between. Note also BUG_ON()
checks at the start.

> So I think its solid, but you failed to mention one state transition,
> which seems ok in any case.

Great, thanks for review.

I'll send the actual patch on top of your changes.

Oleg.