linux-kernel - Re: [PATCH v3 4/4] rcu: Unlock non-start node only after accessing its gp_seq

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Message-ID: <20180522000734.GD40541@joelaf.mtv.corp.google.com>
Date:   Mon, 21 May 2018 17:07:34 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Joel Fernandes <joelaf@...gle.com>, linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>, byungchul.park@....com,
        kernel-team@...roid.com
Subject: Re: [PATCH v3 4/4] rcu: Unlock non-start node only after accessing
 its gp_seq_needed

On Mon, May 21, 2018 at 04:25:38PM -0700, Paul E. McKenney wrote:
> On Sun, May 20, 2018 at 09:42:20PM -0700, Joel Fernandes wrote:
> > We acquire gp_seq_needed locklessly. To be safe, lets do the unlocking
> > after the access.
> 
> Actually, no, we hold rnp_start's ->lock throughout.  And this CPU (or in
> the case of no-CBs CPUs, this task) is in charge of rdp->gp_seq_needed,
> so nothing else is accessing it.  Or at least that is the intent.  ;-)

I was talking about protecting the internal node's rnp->gp_seq_needed, not
the rnp_start's gp_seq_needed.

We are protecting them in the loop:

like this:
for(...)
	if (rnp != rnp_start)
		raw_spin_lock_rcu_node(rnp);
	[...]
	// access rnp->gp_seq and rnp->gp_seq_needed
	[...]
	if (rnp != rnp_start)
		raw_spin_unlock_rcu_node(rnp);

But we don't need to do such protection in unlock_out ? I'm sorry if I'm
missing something, but I'm wondering if rnp->gp_seq_needed of an internal
node can be accessed locklessly, then why can't that be done also in the
funnel locking loop - after all we are holding the rnp_start's lock through
out right?

thanks!

 - Joel