linux-kernel - Re: [PATCH 0/3] virtio/vringh: kill off ACCESS

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Message-ID: <20161125161709.GQ3092@twins.programming.kicks-ass.net>
Date:   Fri, 25 Nov 2016 17:17:09 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>, dbueso@...e.de,
        jasowang@...hat.com, KVM list <kvm@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        virtualization@...ts.linux-foundation.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE()

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().