On Thu, Nov 24, 2016 at 10:36:58PM +0200, Michael S. Tsirkin wrote:
> On Thu, Nov 24, 2016 at 10:25:11AM +0000, Mark Rutland wrote:
> > 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?
> 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.
Thanks,
Mark.
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