Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S932502AbcKYQR0 (ORCPT <rfc822;w@1wt.eu>);
        Fri, 25 Nov 2016 11:17:26 -0500
Received: from bombadil.infradead.org ([198.137.202.9]:56217 "EHLO
        bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S932463AbcKYQRQ (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 25 Nov 2016 11:17:16 -0500
Date: Fri, 25 Nov 2016 17:17:09 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Dmitry Vyukov <dvyukov@google.com>, Boqun Feng <boqun.feng@gmail.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Davidlohr Bueso <dave@stgolabs.net>, dbueso@suse.de,
        jasowang@redhat.com, KVM list <kvm@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        virtualization@lists.linux-foundation.org,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE()
Message-ID: <20161125161709.GQ3092@twins.programming.kicks-ass.net>
References: <1479983114-17190-1-git-send-email-mark.rutland@arm.com>
 <20161124222357-mutt-send-email-mst@kernel.org>
 <20161125112203.GA26611@leverpostej>
 <32dfca07-59f3-b75a-3154-cf6b6c8538f0@de.ibm.com>
 <20161125122356.GB26611@leverpostej>
 <20161125124044.GN3092@twins.programming.kicks-ass.net>
 <20161125124404.GI3174@twins.programming.kicks-ass.net>
 <20161125145512.GA4014@Boquns-MacBook-Pro.local>
 <CACT4Y+ZpzFhmSqOG+dG7QHSNObWatLOjPjNK2BznnRLeRQpF8A@mail.gmail.com>
 <20161125161004.GA30181@leverpostej>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20161125161004.GA30181@leverpostej>
User-Agent: Mutt/1.5.23.1 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 616
Lines: 18

On Fri, Nov 25, 2016 at 04:10:04PM +0000, Mark Rutland wrote:
> On Fri, Nov 25, 2016 at 04:21:39PM +0100, Dmitry Vyukov wrote:

> > What are use cases for such primitive that won't be OK with "read once
> > _and_ atomically"?
> 
> I have none to hand.

Whatever triggers the __builtin_memcpy() paths, and even the size==8
paths on 32bit.

You could put a WARN in there to easily find them.

The advantage of introducing the SINGLE_{LOAD,STORE}() helpers is that
they compiletime validate this the size is 'right' and can runtime check
alignment constraints.

IE, they are strictly stronger than {READ,WRITE}_ONCE().