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 S1755170AbbIRP50 (ORCPT <rfc822;w@1wt.eu>);
	Fri, 18 Sep 2015 11:57:26 -0400
Received: from e39.co.us.ibm.com ([32.97.110.160]:52488 "EHLO
	e39.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1754425AbbIRP5W (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 18 Sep 2015 11:57:22 -0400
X-Helo: d03dlp02.boulder.ibm.com
X-MailFrom: paulmck@linux.vnet.ibm.com
X-RcptTo: linux-kernel@vger.kernel.org
Date: Fri, 18 Sep 2015 08:57:13 -0700
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Will Deacon <will.deacon@arm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>, Peter Zijlstra <peterz@infradead.org>,
        Oleg Nesterov <oleg@redhat.com>,
        "ebiederm@xmission.com" <ebiederm@xmission.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, "mhocko@suse.cz" <mhocko@suse.cz>,
        LKML <linux-kernel@vger.kernel.org>,
        "ktsan@googlegroups.com" <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: <20150918155713.GN4029@linux.vnet.ibm.com>
Reply-To: paulmck@linux.vnet.ibm.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>
 <CACT4Y+a_cBWk9E04EbsgGp6XPr-bQ=Wnt2qMgwiJJq0LSOQ02w@mail.gmail.com>
 <20150918092820.GA27377@arm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150918092820.GA27377@arm.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
X-TM-AS-MML: disable
X-Content-Scanned: Fidelis XPS MAILER
x-cbid: 15091815-0033-0000-0000-000005EFCDE2
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Fri, Sep 18, 2015 at 10:28:20AM +0100, Will Deacon wrote:
> On Fri, Sep 18, 2015 at 10:06:46AM +0100, Dmitry Vyukov wrote:
> > On Fri, Sep 18, 2015 at 10:51 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > > As an alternative patch, could we not do:
> > >
> > >   void put_pid(struct pid *pid)
> > >   {
> > >         struct pid_namespace *ns;
> > >
> > >         if (!pid)
> > >                 return;
> > >
> > >         ns = pid->numbers[pid->level].ns;
> > >         if ((atomic_read(&pid->count) == 1) ||
> > >              atomic_dec_and_test(&pid->count)) {
> > >
> > > +               smp_read_barrier_depends(); /* ctrl-dep */
> > >
> > >                 kmem_cache_free(ns->pid_cachep, pid);
> > >                 put_pid_ns(ns);
> > >         }
> > >   }
> > >
> > > That would upgrade the atomic_read() path to a full READ_ONCE_CTRL(),
> > > and thereby avoid any of the kmem_cache_free() stores from leaking out.
> > > And its free, except on Alpha. Whereas the atomic_read_acquire() will
> > > generate a full memory barrier on whole bunch of archs.
> > 
> > 
> > What you propose makes sense.
> > 
> > +Will, Paul
> > 
> > Can we have something along the lines of:
> > 
> > #define atomic_read_ctrl(v) READ_ONCE_CTRL(&(v)->counter)
> 
> Funnily enough, I had this exact same discussion off-list yesterday
> afternoon, since I wrote some code relying on a ctrl dependency from
> an atomic_read to an atomic_xchg_relaxed.
> 
> So I guess I'm for the addition, but at the same time, could we make
> atomic_read and atomic_set generic too?

If we have some places that could use it, by all means let's add it!

							Thanx, Paul