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

Greg KH
2023-06-08T09:04:15Z

On Thu, Jun 08, 2023 at 10:52:48AM +0200, Peter Zijlstra wrote:
> 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;

I like the idea, as we need a way to say "don't clean this up, it was
passed to somewhere else" for these types of allocations, but have it
"automatically" cleaned up on the error paths.

I have no say in the naming, though I always disliked the idea of a
pointer being "smart" as they are just a dumb memory register :)

thanks,

greg k-h