Date: Fri, 25 Nov 2016 16:10:04 +0000
From: Mark Rutland <mark.rutland@arm.com>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Boqun Feng <boqun.feng@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        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 04:21:39PM +0100, Dmitry Vyukov wrote:
> 
> READ/WRITE_ONCE imply atomicity. Even if their names don't spell it (a
> function name doesn't have to spell all of its guarantees). Most of
> the uses of READ/WRITE_ONCE will be broken if they are not atomic.

In practice, this is certainly the assumption made by many/most users of
the *_ONCE() accessors.

Looking again, Linus does seem to agree that word-sized accesses should
result in single instructions (and be single-copy atomic) [1], so in
contrast to [2], that's clearly *part* of the point of the *_ONCE()
accessors...

> "Read once but not necessary atomically" is a very subtle primitive
> which is very easy to misuse.

I agree. Unfortunately, Linus does not appear to [2].

> What are use cases for such primitive that won't be OK with "read once
> _and_ atomically"?

I have none to hand.

Thanks,
Mark.

[1] http://lkml.iu.edu/hypermail/linux/kernel/1503.3/02674.html
[2] http://lkml.iu.edu/hypermail/linux/kernel/1503.3/02670.html