Date: Fri, 25 Nov 2016 16:32:21 +0000
From: Mark Rutland <mark.rutland@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
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()
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On Fri, Nov 25, 2016 at 05:17:09PM +0100, Peter Zijlstra wrote:
> 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.

Lockref, per:

http://lkml.iu.edu/hypermail/linux/kernel/1503.3/02294.html

In that specific case, a torn value just means we'll retry until we get
a non torn value, due to the cmpxchg. For that case, all we need is the
value to be reloaded per invocation of READ_ONCE().

This guy seems to have the full story:

http://lkml.iu.edu/hypermail/linux/kernel/1503.3/02389.html
http://lkml.iu.edu/hypermail/linux/kernel/1503.3/02558.html

Thanks,
Mark.