Re: [PATCH 01/13] compiler.h: Split {READ,WRITE}_ONCE definitions out into rwonce.h

[Arnd Bergmann]

On Mon, Nov 11, 2019 at 9:10 AM Christian Borntraeger
<borntraeger@xxxxxxxxxx> wrote:
> On 08.11.19 20:57, Arnd Bergmann wrote:
> > On Fri, Nov 8, 2019 at 6:01 PM Will Deacon <will@xxxxxxxxxx> wrote:
> >>
> >> In preparation for allowing architectures to define their own
> >> implementation of the 'READ_ONCE()' macro, move the generic
> >> '{READ,WRITE}_ONCE()' definitions out of the unwieldy 'linux/compiler.h'
> >> and into a new 'rwonce.h' header under 'asm-generic'.
> >
> > Adding Christian BorntrÃger to Cc, he originally added the
> > READ_ONCE()/WRITE_ONCE()
> > code.
> >
> > I wonder if it would be appropriate now to revert back to a much simpler version
> > of these helpers for any modern compiler. As I understand, only gcc-4.6 and
> > gcc4.7 actually need the song-and-dance version with the union and switch/case,
> > while for others, we can might be able back to a macro doing a volatile access.
>
> As far as I know this particular issue with  volatile access on aggregate types
> was fixed in gcc 4.8. On the other hand we know that the current construct will
> work on all compilers. Not so sure about the orignal ACCESS_ONCE implementation.

I've seen problems with clang on the current version, leading to unnecessary
temporaries being spilled to the stack in some cases, so I think it would still
help to simplify it.

We probably don't want the exact ACCESS_ONCE() implementation back
that existed before, but rather something that implements the stricter
READ_ONCE() and WRITE_ONCE(). I'd probably also want to avoid the
__builtin_memcpy() exception for odd-sized accesses and instead have
a separate way to do those.

      Arnd