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

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Date:	Thu, 21 Jul 2016 19:34:36 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, 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 07/20, Paul E. McKenney wrote:
>
> On Wed, Jul 20, 2016 at 07:16:03PM +0200, Oleg Nesterov wrote:
>
> > Now, suppose we add the additional enter/exit's:
> >
> > 	freeze_super(sb)
> > 	{
> > 		// this doesn't block
> > 		__rcu_sync_enter(SEM3);
> > 		__rcu_sync_enter(SEM2);
> > 		__rcu_sync_enter(SEM1);
> >
> > 		down_write(&sb->s_umount);
> > 		if (NEED_TO_FREEZE) {
> > 			percpu_down_write(SEM1);
>
> The above waits for the grace period initiated by __rcu_sync_enter(),
> correct?  Presumably "yes", because it will invoke rcu_sync_enter(), which
> will see the state as GP_ENTER, and will thus wait.

But if down_write() blocks and/or NEED_TO_FREEZE takes some time it
could already see the GP_PASSED state, or at least it can sleep less.

> But your point is that if !NEED_TO_FREEZE, we will get here without
> waiting for a grace period.
>
> But why aren't the __rcu_sync_enter() and rcu_sync_exit() calls inside
> the "if" statement?

Yes, if we do __rcu_sync_enter() inside "if", then rcu_sync_exit() can't
hit GP_ENTER.

But why we should disallow this use-case? It does not complicate the code
at all.

And see above, we want to initiate the GP "asap", so that we will sleep
less later. Although yes, freeze_super() is not the best example. And
__cgroup_procs_write() too, but note that cgroup_kn_lock_live() is rather
heavy, takes the global locks, and can fail. So (ignoring the fact we
are going to switch cgroup_threadgroup_rwsem into the slow mode for now)
__rcu_sync_enter() at the start could help to lessen the time
percpu_down_write(cgroup_threadgroup_rwsem) sleeps with the cgroup_mutex
held.

> That aside, would it make sense to name __rcu_sync_enter() something
> like rcu_sync_begin_to_enter(), rcu_sync_pre_enter() or some such?
> Something to make it clear that it just starts the job and that something
> else is needed to finish it.

Sure. Agreed, will rename.

> And here is an updated state table.  I do not yet separately call out
> __rcu_sync_enter(), though without it the rcu_sync_exit() transition
> out of state B cannot happen.

Thanks! I'll try to double-check it.

Oleg.