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

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Date: Fri, 22 Mar 2024 14:00:42 +0100
From: Markus Elfring <Markus.Elfring@....de>
To: Dan Williams <dan.j.williams@...el.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
 kernel-janitors@...r.kernel.org
Cc: Bjorn Helgaas <bhelgaas@...gle.com>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
 Ira Weiny <ira.weiny@...el.com>,
 Jesse Brandeburg <jesse.brandeburg@...el.com>,
 Jonathan Cameron <Jonathan.Cameron@...wei.com>,
 Julia Lawall <Julia.Lawall@...ia.fr>, Lukas Wunner <lukas.wunner@...el.com>
Subject: Re: [PATCH] cleanup: Add usage and style documentation

…
> +++ b/include/linux/cleanup.h
> @@ -4,6 +4,118 @@
>
>  #include <linux/compiler.h>
>
> +/**
> + * DOC: scope-based cleanup helpers
> + *
> + * The "goto error" pattern is notorious for introducing …

Will any other label become more helpful for this description approach?

> + * this tedium and can aid in maintaining FILO (first in last out)
             ⬆
Would an other word be more appropriate here?

> + * contraindicates a pattern like the following:

I would prefer an other wording approach.

> + *	struct pci_dev *dev __free(pci_dev_put) = NULL;

Programmers got used to null pointer initialisations.

> + * 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.

Can capabilities be clarified better for the applied compilers?

>                                                      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.

I suggest to reconsider such a development concern in more detail.

> + *	struct pci_dev *dev __free(pci_dev_put) =
> + *		pci_get_slot(parent, PCI_DEVFN(0, 0));
> + *
> + * ...which implies that declaring variables in mid-function scope is
> + * not only allowed, but expected.

* Is there a need to separate the ellipsis from the subsequent word
  by a space character?

* You propose a variable definition without specifying extra curly brackets
  (for another compound statement / code block).
  This can work only if an appropriate pointer is returned by the called function.

* The involved identifiers can occasionally get longer.
  Further code layout challenges would need corresponding clarifications.
  How will the handling of line length concerns evolve?

* I suggest to take another look also at the transformation pattern
  “Reduce Scope of Variable”.
  https://refactoring.com/catalog/reduceScopeOfVariable.html

> + * Conversions of existing code to use cleanup helpers should convert
> + * all resources so that no "goto" unwind statements remain. If not all
> + * resources are amenable to cleanup then additional refactoring is
> + * needed to build helper functions, or the function is simply not a
> + * good candidate for conversion.

* How do you think about to specify any more resource cleanup functions
  for growing usage of “smart pointers”?

* Would you like to extend the specification of function pairs for
  improved applications of guard variants?

Regards,
Markus