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 S932614AbbJNQe6 (ORCPT <rfc822;w@1wt.eu>);
	Wed, 14 Oct 2015 12:34:58 -0400
Received: from bombadil.infradead.org ([198.137.202.9]:50297 "EHLO
	bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S932113AbbJNQe4 (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Wed, 14 Oct 2015 12:34:56 -0400
Date: Wed, 14 Oct 2015 18:34:23 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Dmitry Vyukov <dvyukov@google.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        tip-bot for Andrey Ryabinin <tipbot@zytor.com>,
        "linux-tip-commits@vger.kernel.org" 
	<linux-tip-commits@vger.kernel.org>,
        kasan-dev <kasan-dev@googlegroups.com>, Ingo Molnar <mingo@kernel.org>,
        Kostya Serebryany <kcc@google.com>, Borislav Petkov <bp@alien8.de>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sasha Levin <sasha.levin@oracle.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Wolfram Gloger <wmglo@dent.med.uni-muenchen.de>,
        Andrey Konovalov <andreyknvl@google.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Alexander Potapenko <glider@google.com>
Subject: Re: [tip:locking/urgent] compiler, atomics: Provide
 READ_ONCE_NOCHECK ()
Message-ID: <20151014163423.GI3816@twins.programming.kicks-ass.net>
References: <1444750088-24444-2-git-send-email-aryabinin@virtuozzo.com>
 <tip-4115ffdf4d6f8986a7abe1dd522c163f599bc0e6@git.kernel.org>
 <20151014154532.GV3910@linux.vnet.ibm.com>
 <CACT4Y+bEYJJR+CRdngiprjQJ6ykDfsYS3jQL4sTPF1=BoQLvdg@mail.gmail.com>
 <20151014160146.GW3910@linux.vnet.ibm.com>
 <CACT4Y+YKnBQQ434c9wn5C9_aUhLA9shR1iNE34CX7XHcduK2Bg@mail.gmail.com>
 <20151014161615.GG3816@twins.programming.kicks-ass.net>
 <CACT4Y+Zszzbv1kXJfL=axrMPXzX7iDC0=F0YTb2VWpifVRGKhA@mail.gmail.com>
 <20151014162055.GH3816@twins.programming.kicks-ass.net>
 <CALCETrVZP4myE3hvFmgvVcQhKMNvtOUzghC7bjDDMnjbr2Kdhg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CALCETrVZP4myE3hvFmgvVcQhKMNvtOUzghC7bjDDMnjbr2Kdhg@mail.gmail.com>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, Oct 14, 2015 at 09:23:33AM -0700, Andy Lutomirski wrote:
> On Wed, Oct 14, 2015 at 9:20 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Wed, Oct 14, 2015 at 06:18:58PM +0200, Dmitry Vyukov wrote:
> >>
> >> Well, if another thread writes it byte-by-byte, it pretty much does
> >> not matter how you read it.
> >> Note that I said "at least one access is not atomic". If both are
> >> atomic, then this is, of course, legal. And KTSAN considers
> >> READ/WRITE_ONCE as atomic operations.
> >
> > OK, then I'm confused on what exactly the annotation does, but less
> > worried.
> 
> The annotation says "hey, KASAN (etc), don't worry if you think that
> the memory being accessed is out of bounds".  Presumably KTSAN is okay
> with the operation because it's atomic, but KASAN dislikes it because
> it's accessing memory that is out of bounds from the perspective of a
> C program.

There's going to be more of that..

> I'd still rather find a way to just delete get_wchan, but whatever.

:-)