[PATCH 0/3] virtio/vringh: kill off ACCESS_ONCE()

Mark Rutland
Fri, 25 Nov 2016 16:32:21 -0000

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.