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

From: Joe Perches <joe@perches.com>
To: Andrew Morton <akpm@linux-foundation.org>,
	Peter Zijlstra <peterz@infradead.org>
Cc: "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>,
	Julia Lawall <julia.lawall@lip6.fr>
Subject: Re: [PATCH] checkpatch: Add a --strict test for structs with bool member definitions
Date: Wed, 11 Apr 2018 09:51:26 -0700	[thread overview]
Message-ID: <1c9f185f6086e9d89659f93720a27b660ee17c13.camel@perches.com> (raw)
In-Reply-To: <20180411092959.e666ec443e4d3bb6f43901d7@linux-foundation.org>

(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.

> 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.