Re: [PATCH v2 0/2] Lock and Pointer guards

Peter Zijlstra
2023-06-08T08:53:16Z

On Wed, Jun 07, 2023 at 11:41:01AM +0200, Peter Zijlstra wrote:

> > I'm sure there's something horribly wrong in the above, but my point
> > is that I'd really like this to make naming and conceptual sense.
>
> Right, I hear ya. So the asymmetric case (iow destructor only) could be
> seen as using the copy-constructor.
>
> #define DEFINE_CLASS(name, type, exit, init, init_args...) \
> typedef type class_##name##_t; \
> static inline void class_##name##_destructor(type *this) \
> { type THIS = *this; exit; } \
> static inline type class_##name##_constructor(init_args) \
> { type THIS = init; return THIS; }
>
> #define __INSTANTIATE_VAR(name, var) \
> class_##name##_t var __cleanup(class_##name##_destructor)
>
> #define INSTANTIATE_CLASS(name, var) \
> __INSTANTIATE_VAR(name, var) = class_##name##_constructor
>
>
> DEFINE_CLASS(fd, struct fd, fdput(THIS), f, struct fd f)
>
> INSTANTIATE_CLASS(fd, f)(perf_fget_light(fd));
>
>
> Alternatively, you be OK with exposing INSTANTIATE_VAR() to easily
> circumvent the default constructor?

Or perhaps use the smart-pointer concept applied to our classes like:

#define smart_ptr(name, var) \
__INSTANTIATE_VAR(name, var)

To mean a pointer that calls the destructor for class 'name'. I think
the nearest thing C++ has is std::unique_ptr<>.

Then we can write:

DEFINE_CLASS(kfree, void *, kfree(THIS), p, void *p)

smart_ptr(kfree, mem) = kzalloc_node(...);
if (!mem)
return -ENOMEM;

object = mem;

// further initiatlize object with error cases etc..

mem = NULL; // success, we keep it.
return object;