Re: [patch] spinlocks: remove 'volatile'

Avi Kivity
2006-07-08T10:18:26Z

Arjan van de Ven wrote:
>
> >
> > > with PCI, and the PCI posting rules, there is no "one" serializing
> > > instruction, you need to know the specifics of the device in
> question to
> > > cause the flush. So at least there is no universal possible
> > > implementation of volatile as you suggest ;-)
> > >
> >
> > A serializing volatile makes it possible to write portable code to
> > access pci mmio. You'd just follow a write with a read or whatever the
> > rules say.
>
> yeah except that the compiler cannot know what to read; reading back the
> same memory location is NOT correct nor safe. It's device specific, for
> some devices it'll be safe, for others you have to read some OTHER
> location.
>

I didn't suggest the compiler could or should do it, just that it would
be possible (for the _user_) to write portable ISO C code to access PCI
mmio registers, if volatile's implementation serialized access.

With the current implementation of volatile in gcc, it is impossible -
you need to resort to inline assembly for some architectures, which is
not an ISO C feature.

And I'm not suggesting that it would be a good idea to use volatile even
if it was corrected - it would have to take a worst-case approach and
thus would generate very bad code.

--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.