On 11/25/2016 12:22 PM, Mark Rutland wrote:

On Thu, Nov 24, 2016 at 10:36:58PM +0200, Michael S. Tsirkin wrote:

quoted

On Thu, Nov 24, 2016 at 10:25:11AM +0000, Mark Rutland wrote:

quoted

For several reasons, it would be beneficial to kill off ACCESS_ONCE()
tree-wide, in favour of {READ,WRITE}_ONCE(). These work with aggregate types,
more obviously document their intended behaviour, and are necessary for tools
like KTSAN to work correctly (as otherwise reads and writes cannot be
instrumented separately).

While it's possible to script the bulk of this tree-wide conversion, some cases
such as the virtio code, require some manual intervention. This series moves
the virtio and vringh code over to {READ,WRITE}_ONCE(), in the process fixing a
bug in the virtio headers.

Thanks,
Mark.

I don't have a problem with this specific patchset.

Good to hear. :)

Does that mean you're happy to queue these patches? Or would you prefer
a new posting at some later point, with ack/review tags accumulated?

quoted

Though I really question the whole _ONCE APIs esp with
aggregate types - these seem to generate a memcpy and
an 8-byte read/writes sometimes, and I'm pretty sure this simply
can't be read/written at once on all architectures.

Yes, in cases where the access is larger than the machine can perform in
a single access, this will result in a memcpy.

My understanding is that this has always been the case with
ACCESS_ONCE(), where multiple accesses were silently/implicitly
generated by the compiler.

We could add some compile-time warnings for those cases. I'm not sure if
there's a reason we avoided doing that so far; perhaps Christian has a
some idea.

My first version had this warning, but it was removed later on as requested
by Linus

http://lkml.iu.edu/hypermail/linux/kernel/1503.3/02670.html
---snip---

Get rid of the f*cking size checks etc on READ_ONCE() and friends.

They are about - wait for it - "reading a value once".

Note how it doesn't say ANYTHING about "atomic" or anything like that.
It's about reading *ONCE*.

---snip---