Re: [Xen-devel] [RFC PATCH] docs: add README.atomic

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 14 Jun 2017, Jan Beulich wrote:
> > +What ACCESS_ONCE does *not* guarantee though is this access is done in a
> > +single instruction, so complex or non-native or unaligned data types are
> > +not guaranteed to be atomic. If for instance counter would be a 64-bit 
> > value
> > +on a 32-bit system, the compiler would probably generate two load 
> > instructions,
> > +which could end up in reading a wrong value if some other CPU changes the 
> > other
> > +half of the variable in between those two reads.
> > +However accessing _aligned and native_ data types is guaranteed to be 
> > atomic
> > +in the architectures supported by Xen, so ACCESS_ONCE is safe to use when
> > +these conditions are met.
> 
> As mentioned before, such a guarantee does not exist. Please only
> state what is really the case, i.e. we _expect_ compilers to behave
> this way.

Regarding compilers support: do we state clearly in any docs or website
what are the compilers we actually support? I think this would be the
right opportunity to do it.

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