linux-kernel - Re: [PATCH] checkpatch: Add a --strict test for structs with bool member definitions

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Date:   Sat, 14 Apr 2018 23:19:09 +0200 (CEST)
From:   Julia Lawall <julia.lawall@...6.fr>
To:     Joe Perches <joe@...ches.com>
cc:     Julia Lawall <julia.lawall@...6.fr>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andy Whitcroft <apw@...onical.com>, yuankuiz@...eaurora.org,
        Linux PM <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        paulmck@...ux.vnet.ibm.com, Ingo Molnar <mingo@...nel.org>,
        Len Brown <len.brown@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] checkpatch: Add a --strict test for structs with bool
 member definitions



On Wed, 11 Apr 2018, Joe Perches wrote:

> On Thu, 2018-04-12 at 08:22 +0200, Julia Lawall wrote:
> > On Wed, 11 Apr 2018, Joe Perches wrote:
> > > On Wed, 2018-04-11 at 09:29 -0700, Andrew Morton wrote:
> > > > We already have some 500 bools-in-structs
> > >
> > > I got at least triple that only in include/
> > > so I expect there are at probably an order
> > > of magnitude more than 500 in the kernel.
> > >
> > > I suppose some cocci script could count the
> > > actual number of instances.  A regex can not.
> >
> > I got 12667.
>
> Could you please post the cocci script?
>
> > I'm not sure to understand the issue.  Will using a bitfield help if there
> > are no other bitfields in the structure?
>
> IMO, not really.
>
> The primary issue is described by Linus here:
> https://lkml.org/lkml/2017/11/21/384
>
> I personally do not find a significant issue with
> uncontrolled sizes of bool in kernel structs as
> all of the kernel structs are transitory and not
> written out to storage.
>
> I suppose bool bitfields are also OK, but for the
> RMW required.
>
> Using unsigned int :1 bitfield instead of bool :1
> has the negative of truncation so that the uint
> has to be set with !! instead of a simple assign.

At least with gcc 5.4.0, a number of structures become larger with
unsigned int :1. bool:1 seems to mostly solve this problem.  The structure
ichx_desc, defined in drivers/gpio/gpio-ich.c seems to become larger with
both approaches.

julia