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

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Date:   Wed, 11 Apr 2018 19:00:49 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew Morton <akpm@...ux-foundation.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, Apr 11, 2018 at 09:29:59AM -0700, Andrew Morton wrote:
> 
> OK.  I guess.  But I'm not really seeing some snappy description which
> helps people understand why checkpatch is warning about this. 

 "Results in architecture dependent layout."

is the best short sentence I can come up with.

> 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.

I still have room in my /dev/null mailbox for pure checkpatch patches.

> (ooh, https://lkml.org/lkml/2017/11/21/384 is working this morning)

Yes, we really should not use lkml.org for references. Sadly google
displays it very prominently when you search for something.

> 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.

So that is true of u8 on Alpha <EV56 too. If you want concurrent, you
had better know what you're doing.

> 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(?).

The smallest addressable type in C is a byte, so while _Bool may be
larger than a byte, it cannot be smaller. Otherwise we could not write:

	_Bool var;
	_Boll *ptr = &var;

Which is something that comes apart with bitfields.