RE: [PATCH] cleanup: Add usage and style documentation

[Tian, Kevin]

> From: Dan Williams <dan.j.williams@xxxxxxxxx>
> Sent: Thursday, March 21, 2024 6:05 AM
> + *
> + * Note that unwind order is dictated by declaration order. That
> + * contraindicates a pattern like the following:
> + *
> + * .. code-block:: c
> + *
> + *	int num, ret = 0;
> + *	struct pci_dev *bridge = ctrl->pcie->port;
> + *	struct pci_bus *parent = bridge->subordinate;
> + *	struct pci_dev *dev __free(pci_dev_put) = NULL;
> + *
> + *	pci_lock_rescan_remove();
> + *
> + *	dev = pci_get_slot(parent, PCI_DEVFN(0, 0));
> + *
> + * In this case @dev is declared in x-mas tree style in a preamble
> + * declaration block. That is problematic because it destroys the
> + * compiler's ability to infer proper unwind order. If other cleanup
> + * helpers appeared in such a function that depended on @dev being live
> + * to complete their unwind then using the "struct obj_type *obj
> + * __free(...) = NULL" style is an anti-pattern that potentially causes
> + * a use-after-free bug. Instead, the expectation is this conversion:
> + *

an example of dependent cleanup helpers might be helpful to
better understand this expectation?