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

From: Julia Lawall <julia.lawall@lip6.fr>
To: Joe Perches <joe@perches.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Andy Whitcroft <apw@canonical.com>,
	yuankuiz@codeaurora.org, Linux PM <linux-pm@vger.kernel.org>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	paulmck@linux.vnet.ibm.com, Ingo Molnar <mingo@kernel.org>,
	Len Brown <len.brown@intel.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] checkpatch: Add a --strict test for structs with bool member definitions
Date: Thu, 12 Apr 2018 08:22:12 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.20.1804120816130.3559@hadrien> (raw)
In-Reply-To: <1c9f185f6086e9d89659f93720a27b660ee17c13.camel@perches.com>

On Wed, 11 Apr 2018, Joe Perches wrote:

> (Adding Julia Lawall)
>
> 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.

I'm not sure to understand the issue.  Will using a bitfield help if there
are no other bitfields in the structure?

julia

>
> > and the owners of that code will
> > be wondering whether they should change them, and whether they should
> > apply those remove-bool-in-struct patches which someone sent them.
>
> Which is why the warning is --strict only
>
> > So... can we please get some clarity here?
>
>
> > ...
> >
> > (ooh, https://lkml.org/lkml/2017/11/21/384 is working this morning)
> >
> > hm, Linus suggests that instead of using
> >
> > 	bool mybool;
> >
> > we should use
> >
> > 	unsigned mybool:1;
> >
> > However that introduces the risk that alterations of mybool will use
> > nonatomic rmw operations.
> >
> > 	unsigned myboolA:1;
> > 	unsigned myboolB:1;
> >
> > so
> >
> > 	foo->myboolA = 1;
> >
> > could scribble on concurrent alterations of foo->myboolB.  I think.
>
> Without barriers, that could happen anyway.
>
> To me, the biggest problem with conversions
> from bool to bitfield is logical.  ie:
>
> 	unsigned int.singlebitfield = 4;
>
> is not the same result as
>
> 	bool = 4;
>
> because of implicit truncation vs boolean conversion
> so a direct change of bool use in structs to unsigned
> would also require logic analysis.
>
>