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 S1754744AbbIRPdL (ORCPT <rfc822;w@1wt.eu>);
	Fri, 18 Sep 2015 11:33:11 -0400
Received: from mx1.redhat.com ([209.132.183.28]:48009 "EHLO mx1.redhat.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1751659AbbIRPdJ (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Fri, 18 Sep 2015 11:33:09 -0400
Date: Fri, 18 Sep 2015 17:30:12 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dmitry Vyukov <dvyukov@google.com>, ebiederm@xmission.com,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>, mhocko@suse.cz,
        LKML <linux-kernel@vger.kernel.org>, ktsan@googlegroups.com,
        Kostya Serebryany <kcc@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        Alexander Potapenko <glider@google.com>,
        Hans Boehm <hboehm@google.com>
Subject: Re: [PATCH] kernel: fix data race in put_pid
Message-ID: <20150918153012.GA15521@redhat.com>
References: <1442496268-47803-1-git-send-email-dvyukov@google.com> <20150917160837.GA26050@redhat.com> <CACT4Y+YKmhz-+sugKt86CF2715YtoVz4ee+sRfDp2Wu==A4zPQ@mail.gmail.com> <20150917174456.GA30178@redhat.com> <CACT4Y+bnqMGjMV6FHHJWsx0YaxWmRqAM+YY7b5wz-7JmFBnwjA@mail.gmail.com> <20150917180919.GA32116@redhat.com> <20150918085156.GS3816@twins.programming.kicks-ass.net> <20150918132844.GA10241@redhat.com> <20150918134630.GW3816@twins.programming.kicks-ass.net>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150918134630.GW3816@twins.programming.kicks-ass.net>
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

I can't resist. You certainly don't need this spam, but

On 09/18, Peter Zijlstra wrote:
>
> On Fri, Sep 18, 2015 at 03:28:44PM +0200, Oleg Nesterov wrote:
> >
> > And again, I simply can't understand if this code
> >
> > 	if (READ_ONCE_CTRL(Y))
> > 		BUG_ON(X == 0);
> >
> > to me it does _not_ look correct in theory.
>
> So control dependencies provide a load-store barrier. Your examples
> above rely on a load-load barrier; BUG_ON(X == 0) is a load.

Yes, and this is just obvious.

Yet I was confused because reading other emails I misunderstood the
proposed semantics of atomic_read_ctrl(), so I started to suspect
that in fact _CTRL() does more than I used to think.

Nevermind, sorry for noise, thanks to all.

Oleg.