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SubjectRe: linux-next: build failure


On Tue, 29 Jul 2008, Ingo Molnar wrote:
>
> > @@ -287,7 +287,7 @@ static inline const cpumask_t *get_cpu_mask(unsigned int cpu)
> > * gcc optimizes it out (it's a constant) and there's no huge stack
> > * variable created:
> > */
> > -#define cpumask_of_cpu(cpu) ({ *get_cpu_mask(cpu); })
> > +#define cpumask_of_cpu(cpu) (*get_cpu_mask(cpu))
>
> hm, i'm wondering - is this a compiler bug?

Not necessarily. The code is fragile.

Doing a statement expression basically creates a new temporary variable
with pretty much undefined scope. Taking the address of it *may* be
optimized away to the original cpu_mask pointer, but it's not really a
well-defined operation: there really _is_ a local temporary variable, and
if you were to change things through the address-of thing, gcc would be
buggy if it had done the optimization.

So the "address-of statement expression" really is a dubious construct.

That said, the change that Stephen introduces is _not_ a no-op. It very
fundamentally changes the code - exactly because now there is no temporary
value created any more: it's a real lvalue, and now anybody who does
&cpumask_of_cpu(cpu) will fundamentally get the original pointer back,
except it has lost the "const".

And that's actually dangerous - exactly because it now loses the "const"
thing, there may be people who end up modifying the result without
getting any compile-time warnings.
I would _seriously_ suggest that both the original code and Stephen's
modified version is really bad. And you should have taken my interface
as-is - one that returns a "const cpumask_t *", and nothing else.

Anything else is simply fundamentally broken.

Linus


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