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Date:   Thu, 8 Jun 2023 10:52:48 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     keescook@chromium.org, gregkh@linuxfoundation.org,
        pbonzini@redhat.com, linux-kernel@vger.kernel.org,
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Subject: Re: [PATCH v2 0/2] Lock and Pointer guards
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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;