linux-kernel - Re: [PATCH v4] coding-style: Clarify the expectations around bool

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Message-ID: <20190113164936.GW6310@bombadil.infradead.org>
Date:   Sun, 13 Jan 2019 08:49:36 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Jason Gunthorpe <jgg@...lanox.com>
Cc:     Joe Perches <joe@...ches.com>, Gal Pressman <galpress@...zon.com>,
        Bart Van Assche <bvanassche@....org>,
        Stephen Warren <swarren@...dotorg.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        "xavier.huwei@...wei.com" <xavier.huwei@...wei.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        Doug Ledford <dledford@...hat.com>,
        Stephen Warren <swarren@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] coding-style: Clarify the expectations around bool

On Thu, Jan 10, 2019 at 11:48:13PM +0000, Jason Gunthorpe wrote:
> +The Linux kernel bool type is an alias for the C99 _Bool type. bool values can
> +only evaluate to 0 or 1, and implicit or explicit conversion to bool
> +automatically converts the value to true or false. When using bool types the
> +!! construction is not needed, which eliminates a class of bugs.
> +
> +When working with bool values the true and false definitions should be used
> +instead of 0 and 1.
> +
> +bool function return types and stack variables are always fine to use whenever
> +appropriate. Use of bool is encouraged to improve readability and is often a
> +better option than 'int' for storing boolean values.

It's awkward to start a sentence with a lower case letter.  How about
rephrasing this paragraph and the following one as:

  Using bool as the return type of a function or as a variable is always
  fine when appropriate.  It often improves readability and is a better option
  than int for storing boolean values.  Using bool in data structures is
  more debatable; its size and alignment can vary between architectures.

> +Do not use bool if cache line layout or size of the value matters, its size
> +and alignment varies based on the compiled architecture. Structures that are
> +optimized for alignment and size should not use bool.
> +
> +If a structure has many true/false values, consider consolidating them into a
> +bitfield with 1 bit members, or using an appropriate fixed width type, such as
> +u8.