From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1751915AbbJGHvU (ORCPT <rfc822;w@1wt.eu>);
	Wed, 7 Oct 2015 03:51:20 -0400
Received: from casper.infradead.org ([85.118.1.10]:50648 "EHLO
	casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751887AbbJGHvR (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 7 Oct 2015 03:51:17 -0400
Date: Wed, 7 Oct 2015 09:51:14 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com,
        dipankar@in.ibm.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
        tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com,
        edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com,
        oleg@redhat.com, bobby.prani@gmail.com
Subject: Re: [PATCH tip/core/rcu 02/18] rcu: Move rcu_report_exp_rnp() to
 allow consolidation
Message-ID: <20151007075114.GW2881@worktop.programming.kicks-ass.net>
References: <20151006162907.GA12020@linux.vnet.ibm.com>
 <1444148977-14108-1-git-send-email-paulmck@linux.vnet.ibm.com>
 <1444148977-14108-2-git-send-email-paulmck@linux.vnet.ibm.com>
 <20151006202937.GX3604@twins.programming.kicks-ass.net>
 <20151006205850.GW3910@linux.vnet.ibm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20151006205850.GW3910@linux.vnet.ibm.com>
User-Agent: Mutt/1.5.22.1 (2013-10-16)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Tue, Oct 06, 2015 at 01:58:50PM -0700, Paul E. McKenney wrote:
> On Tue, Oct 06, 2015 at 10:29:37PM +0200, Peter Zijlstra wrote:
> > On Tue, Oct 06, 2015 at 09:29:21AM -0700, Paul E. McKenney wrote:
> > > +static void __maybe_unused rcu_report_exp_rnp(struct rcu_state *rsp,
> > > +					      struct rcu_node *rnp, bool wake)
> > > +{
> > > +	unsigned long flags;
> > > +	unsigned long mask;
> > > +
> > > +	raw_spin_lock_irqsave(&rnp->lock, flags);
> > 
> > Normally we require a comment with barriers, explaining the order and
> > the pairing etc.. :-)
> > 
> > > +	smp_mb__after_unlock_lock();
> 
> Hmmmm...  That is not good.
> 
> Worse yet, I am missing comments on most of the pre-existing barriers
> of this form.

Yes I noticed.. :/

> The purpose is to enforce the heavy-weight grace-period memory-ordering
> guarantees documented in the synchronize_sched() header comment and
> elsewhere.

> They pair with anything you might use to check for violation
> of these guarantees, or, simiarly, any ordering that you might use when
> relying on these guarantees.

I'm sure you know what that means, but I've no clue ;-) That is, I
wouldn't know where to start looking in the RCU implementation to verify
the barrier is either needed or sufficient. Unless you mean _everywhere_
:-)

> I could add something like  "/* Enforce GP memory ordering. */"
> 
> Or perhaps "/* See synchronize_sched() header. */"
> 
> I do not propose reproducing the synchronize_sched() header on each
> of these.  That would be verbose, even for me!  ;-)
> 
> Other thoughts?

Well, this is an UNLOCK+LOCK on non-matching lock variables upgrade to
full barrier thing, right?

To me its not clear which UNLOCK we even match here. I've just read the
sync_sched() header, but that doesn't help me either, so referring to
that isn't really helpful either.

In any case, I don't want to make too big a fuzz here, but I just
stumbled over a lot of unannotated barriers and figured I ought to say
something about it.