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

On Fri, Nov 25, 2016 at 10:07 AM, Mark Rutland [off-list ref] wrote:

On Fri, Nov 25, 2016 at 09:52:50AM -0800, Linus Torvalds wrote:

quoted

READ/WRITE_ONCE() are atomic *WHEN*THAT*IS*POSSIBLE*.

quoted

But sometimes it's not going to be atomic.

That's the problem.

It has never really been much of a problem, and quite frankly, the
solution would never be to add _another_ crazy new function that will
just confuse everybody.

If you have code that depends on atomicity of READ_ONCE() and friends,
then you should add the appropriate built-time assert to *your* code.
Not to some random generic function that others care about and that
others do _not_ have problems with.

So if you have a data structure in virtio that is
architecture-dependent and might not be a word size, you add the

   BUILD_BUG_ON(sizeof(mytype) > sizeof(long));

or whatever. With a big comment saying "this needs to actually fit in
a single register so that we can do atomic accesses".

You do not screw it up for everybody else.

                Linus