linux-kernel - RE: [PATCH] cleanup: Add usage and style documentation

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Message-ID: <BN9PR11MB52764D455B465E7C69F3A4C38C312@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Fri, 22 Mar 2024 05:43:49 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: "Williams, Dan J" <dan.j.williams@...el.com>, "peterz@...radead.org"
	<peterz@...radead.org>, "torvalds@...ux-foundation.org"
	<torvalds@...ux-foundation.org>
CC: Bjorn Helgaas <bhelgaas@...gle.com>, "Weiny, Ira" <ira.weiny@...el.com>,
	Jonathan Cameron <jonathan.cameron@...wei.com>, "Brandeburg, Jesse"
	<jesse.brandeburg@...el.com>, Ilpo Järvinen
	<ilpo.jarvinen@...ux.intel.com>, "Wunner, Lukas" <lukas.wunner@...el.com>,
	Jonathan Corbet <corbet@....net>, "linux-pci@...r.kernel.org"
	<linux-pci@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "gregkh@...uxfoundation.org"
	<gregkh@...uxfoundation.org>, "linux-doc@...r.kernel.org"
	<linux-doc@...r.kernel.org>
Subject: RE: [PATCH] cleanup: Add usage and style documentation

> From: Dan Williams <dan.j.williams@...el.com>
> 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?