cleanup: Add usage and style documentation

When proposing that PCI grow some new cleanup helpers for pci_dev_put()
and pci_dev_{lock,unlock} [1], Bjorn had some fundamental questions
about expectations and best practices. Upon reviewing an updated
changelog with those details he recommended adding them to documentation
in the header file itself.

Add that documentation and link it into the rendering for
Documentation/core-api/.

Signed-off-by: Dan Williams <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Reviewed-by: Jonathan Cameron <[e-mail]>
Reviewed-by: Kevin Tian <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/core-api/cleanup.rst

···
+.. SPDX-License-Identifier: GPL-2.0
+
+===========================
+Scope-based Cleanup Helpers
+===========================
+
+.. kernel-doc:: include/linux/cleanup.h
+   :doc: scope-based cleanup helpers

Documentation/core-api/index.rst

···
 
    kobject
    kref
+   cleanup
    assoc_array
    xarray
    maple_tree

include/linux/cleanup.h

···
 
 #include <linux/compiler.h>
 
+/**
+ * DOC: scope-based cleanup helpers
+ *
+ * The "goto error" pattern is notorious for introducing subtle resource
+ * leaks. It is tedious and error prone to add new resource acquisition
+ * constraints into code paths that already have several unwind
+ * conditions. The "cleanup" helpers enable the compiler to help with
+ * this tedium and can aid in maintaining LIFO (last in first out)
+ * unwind ordering to avoid unintentional leaks.
+ *
+ * As drivers make up the majority of the kernel code base, here is an
+ * example of using these helpers to clean up PCI drivers. The target of
+ * the cleanups are occasions where a goto is used to unwind a device
+ * reference (pci_dev_put()), or unlock the device (pci_dev_unlock())
+ * before returning.
+ *
+ * The DEFINE_FREE() macro can arrange for PCI device references to be
+ * dropped when the associated variable goes out of scope::
+ *
+ *	DEFINE_FREE(pci_dev_put, struct pci_dev *, if (_T) pci_dev_put(_T))
+ *	...
+ *	struct pci_dev *dev __free(pci_dev_put) =
+ *		pci_get_slot(parent, PCI_DEVFN(0, 0));
+ *
+ * The above will automatically call pci_dev_put() if @dev is non-NULL
+ * when @dev goes out of scope (automatic variable scope). If a function
+ * wants to invoke pci_dev_put() on error, but return @dev (i.e. without
+ * freeing it) on success, it can do::
+ *
+ *	return no_free_ptr(dev);
+ *
+ * ...or::
+ *
+ *	return_ptr(dev);
+ *
+ * The DEFINE_GUARD() macro can arrange for the PCI device lock to be
+ * dropped when the scope where guard() is invoked ends::
+ *
+ *	DEFINE_GUARD(pci_dev, struct pci_dev *, pci_dev_lock(_T), pci_dev_unlock(_T))
+ *	...
+ *	guard(pci_dev)(dev);
+ *
+ * The lifetime of the lock obtained by the guard() helper follows the
+ * scope of automatic variable declaration. Take the following example::
+ *
+ *	func(...)
+ *	{
+ *		if (...) {
+ *			...
+ *			guard(pci_dev)(dev); // pci_dev_lock() invoked here
+ *			...
+ *		} // <- implied pci_dev_unlock() triggered here
+ *	}
+ *
+ * Observe the lock is held for the remainder of the "if ()" block not
+ * the remainder of "func()".
+ *
+ * Now, when a function uses both __free() and guard(), or multiple
+ * instances of __free(), the LIFO order of variable definition order
+ * matters. GCC documentation says:
+ *
+ * "When multiple variables in the same scope have cleanup attributes,
+ * at exit from the scope their associated cleanup functions are run in
+ * reverse order of definition (last defined, first cleanup)."
+ *
+ * When the unwind order matters it requires that variables be defined
+ * mid-function scope rather than at the top of the file.  Take the
+ * following example and notice the bug highlighted by "!!"::
+ *
+ *	LIST_HEAD(list);
+ *	DEFINE_MUTEX(lock);
+ *
+ *	struct object {
+ *	        struct list_head node;
+ *	};
+ *
+ *	static struct object *alloc_add(void)
+ *	{
+ *	        struct object *obj;
+ *
+ *	        lockdep_assert_held(&lock);
+ *	        obj = kzalloc(sizeof(*obj), GFP_KERNEL);
+ *	        if (obj) {
+ *	                LIST_HEAD_INIT(&obj->node);
+ *	                list_add(obj->node, &list):
+ *	        }
+ *	        return obj;
+ *	}
+ *
+ *	static void remove_free(struct object *obj)
+ *	{
+ *	        lockdep_assert_held(&lock);
+ *	        list_del(&obj->node);
+ *	        kfree(obj);
+ *	}
+ *
+ *	DEFINE_FREE(remove_free, struct object *, if (_T) remove_free(_T))
+ *	static int init(void)
+ *	{
+ *	        struct object *obj __free(remove_free) = NULL;
+ *	        int err;
+ *
+ *	        guard(mutex)(&lock);
+ *	        obj = alloc_add();
+ *
+ *	        if (!obj)
+ *	                return -ENOMEM;
+ *
+ *	        err = other_init(obj);
+ *	        if (err)
+ *	                return err; // remove_free() called without the lock!!
+ *
+ *	        no_free_ptr(obj);
+ *	        return 0;
+ *	}
+ *
+ * That bug is fixed by changing init() to call guard() and define +
+ * initialize @obj in this order::
+ *
+ *	guard(mutex)(&lock);
+ *	struct object *obj __free(remove_free) = alloc_add();
+ *
+ * Given that the "__free(...) = NULL" pattern for variables defined at
+ * the top of the function poses this potential interdependency problem
+ * the recommendation is to always define and assign variables in one
+ * statement and not group variable definitions at the top of the
+ * function when __free() is used.
+ *
+ * Lastly, given that the benefit of cleanup helpers is removal of
+ * "goto", and that the "goto" statement can jump between scopes, the
+ * expectation is that usage of "goto" and cleanup helpers is never
+ * mixed in the same function. I.e. for a given routine, convert all
+ * resources that need a "goto" cleanup to scope-based cleanup, or
+ * convert none of them.
+ */
+
 /*
  * DEFINE_FREE(name, type, free):
  *	simple helper macro that defines the required wrapper for a __free()