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 S1755801AbbFLTA3 (ORCPT <rfc822;w@1wt.eu>);
	Fri, 12 Jun 2015 15:00:29 -0400
Received: from mx1.redhat.com ([209.132.183.28]:49729 "EHLO mx1.redhat.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1755739AbbFLTA0 (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Fri, 12 Jun 2015 15:00:26 -0400
Date: Fri, 12 Jun 2015 20:59:19 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>, umgwanakikbuti@gmail.com,
        mingo@elte.hu, ktkhai@parallels.com, rostedt@goodmis.org,
        tglx@linutronix.de, juri.lelli@gmail.com, pang.xunlei@linaro.org,
        wanpeng.li@linux.intel.com, linux-kernel@vger.kernel.org,
        Al Viro <viro@ZenIV.linux.org.uk>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 11/18] seqcount: Introduce raw_write_seqcount_barrier()
Message-ID: <20150612185919.GA11558@redhat.com>
References: <20150611124636.448700267@infradead.org> <20150611124743.374180021@infradead.org> <20150611153341.GK3913@linux.vnet.ibm.com> <20150611214557.GA4249@linux.vnet.ibm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150611214557.GA4249@linux.vnet.ibm.com>
User-Agent: Mutt/1.5.18 (2008-05-17)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 06/11, Paul E. McKenney wrote:
>
> > > + *      seqcount_t seq;
> > > + *      bool X = true, Y = false;
> > > + *
> > > + *      void read(void)
> > > + *      {
> > > + *              bool x, y;
> > > + *
> > > + *              do {
> > > + *                      int s = read_seqcount_begin(&seq);
> > > + *
> > > + *                      x = X; y = Y;
> > > + *
> > > + *              } while (read_seqcount_retry(&seq, s));
> > > + *
> > > + *              BUG_ON(!x && !y);
> > > + *      }
> > > + *
> > > + *      void write(void)
> > > + *      {
> > > + *              Y = true;
> > > + *
> > > + *              write_seqcount_begin(seq);
> > > + *              write_seqcount_end(seq);
> > > + *
> > > + *              X = false;
> > > + *      }
> >
> > > +static inline void raw_write_seqcount_barrier(seqcount_t *s)
> > > +{
> > > +	s->sequence++;
> > > +	smp_wmb();
> > > +	s->sequence++;
> > > +}
> > > +
> > >  /*
> > >   * raw_write_seqcount_latch - redirect readers to even/odd copy
> > >   * @s: pointer to seqcount_t
> >
> > Looks good otherwise.
> >
> > Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> Color me slow and stupid.  Maybe due to reviewing a patch too early in
> the morning, who knows?
>
> There is nothing above that prevents the compiler and the CPU from
> reordering the assignments to X and Y with the increment of s->sequence++.

Yes, but this doesn't matter, I think. The writer does

	Y = true;
	1st_increment;

	wmb();

	2nd_increment;
	X = false;

and we do not care about reordering before or after wmnb() at all. But we
rely on the fact that 1st_increment can not be reordered with "X = false",
and that "Y = true" can not be reordered with the 2nd_increment.


And another simple "proof" is that  seqcount_barrier() is equivalent to
write_seqcount_begin() + + write_seqcount_end() and thus the code above
is correct, or the ACQUIRE/RELEASE semantics of seqcount_t is broken ;)

Oleg.