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

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Date:   Wed, 11 Apr 2018 09:29:59 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Joe Perches <joe@...ches.com>,
        "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 10:15:02 +0200 Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Apr 10, 2018 at 03:00:11PM -0700, Andrew Morton wrote:
> > On Tue, 10 Apr 2018 14:53:51 -0700 Joe Perches <joe@...ches.com> wrote:
> > 
> > > On Tue, 2018-04-10 at 14:39 -0700, Andrew Morton wrote:
> > > > On Tue, 10 Apr 2018 11:19:54 -0700 Joe Perches <joe@...ches.com> wrote:
> > > > 
> > > > > A struct with a bool member can have different sizes on various
> > > > > architectures because neither bool size nor alignment is standardized.
> > > > 
> > > > What's wrong with bools in structs?
> > > 
> > > See above.
> > 
> > Yeah, but so what?  `long' has different sizes on different
> > architectures too.
> 
> Right, so we have ILP32/LP64 for all our 32/64 bit archs respectively.
> So only 2 possible variations to consider, and if you know your bitness
> you know your layout.
> 
> (+- some really unfortunate alignment exceptions, the worst of which
> Arnd recently removed, hooray!)
> 
> But neither says anything about sizeof(_Bool), and the standard leaves
> it undefined and only mandates it is large enough to store either 0 or
> 1 (and I suspect this vagueness is because there are architectures that
> either have no byte addressibility or it's more expensive than word
> addressibility).
> 
> Typically GCC chooses a single byte to represent _Bool, but there are no
> guarantees. This means that when you care about structure layout (as we
> all really should) things go wobbly when you use _Bool.
> 
> If GCC were to guarantee a 1 byte _Bool for all Linux ABIs we could
> reconsider.

OK.  I guess.  But I'm not really seeing some snappy description which
helps people understand why checkpatch is warning about this.  We
already have some 500 bools-in-structs 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.

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.

I guess that risk is also present if myboolA and myboolB were `bool',
too.  The compiler could do any old thing with them including, perhaps,
using a single-bit bitfield(?).