Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

[Satyam Sharma <satyam@infradead.org>]

[ Sorry for empty subject line in previous mail. I intended to make
  a patch so cleared it to change it, but ultimately neither made
  a patch nor restored subject line. Done that now. ]


On Thu, 16 Aug 2007, Herbert Xu wrote:

> On Thu, Aug 16, 2007 at 06:06:00AM +0530, Satyam Sharma wrote:
> > 
> > that are:
> > 
> > 	while ((atomic_read(&waiting_for_crash_ipi) > 0) && msecs) {
> > 		mdelay(1);
> > 		msecs--;
> > 	}
> > 
> > where mdelay() becomes __const_udelay() which happens to be in another
> > translation unit (arch/i386/lib/delay.c) and hence saves this callsite
> > from being a bug :-)
> 
> The udelay itself certainly should have some form of cpu_relax in it.

Yes, a form of barrier() must be present in mdelay() or udelay() itself
as you say, having it in __const_udelay() is *not* enough (superflous
actually, considering it is already a separate translation unit and
invisible to the compiler).

However, there are no compiler barriers on the macro-definition-path
between mdelay(1) and __const_udelay(), so the only thing that saves us
from being a bug here is indeed the different-translation-unit concept.