Re: [patch V2 01/10] cleanup: Provide retain_ptr()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Mar 13, 2025 at 02:03:38PM +0100, Thomas Gleixner wrote:
> In cases where an allocation is consumed by another function, the
> allocation needs to be retained on success or freed on failure. The code
> pattern is usually:
> 
> 	struct foo *f = kzalloc(sizeof(*f), GFP_KERNEL);
> 	struct bar *b;
> 
> 	,,,
> 	// Initialize f
> 	...
> 	if (ret)
> 		goto free;
>         ...
> 	bar = bar_create(f);
> 	if (!bar) {
> 		ret = -ENOMEM;
> 	   	goto free;
> 	}
> 	...
> 	return 0;
> free:
> 	kfree(f);
> 	return ret;
> 
> This prevents using __free(kfree) on @f because there is no canonical way
> to tell the cleanup code that the allocation should not be freed.
> 
> Abusing no_free_ptr() by force ignoring the return value is not really a
> sensible option either.
> 
> Provide an explicit macro retain_ptr(), which NULLs the cleanup
> pointer. That makes it easy to analyze and reason about.

So no objection per se, but one way to solve this is by handing
ownership to bar_create(), such that it is responsible for freeing f on
failure.

Anyway, I suspect the __must_check came from Linus, OTOH take_fd(), the
equivalent for file descriptors	also don't have that __must_check. So
clearly we have precedent here.