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Date: Wed, 11 Apr 2018 19:00:49 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Joe Perches <joe@perches.com>, "Rafael J. Wysocki" <rafael@kernel.org>,
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        Ingo Molnar <mingo@kernel.org>, Len Brown <len.brown@intel.com>,
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Subject: Re: [PATCH] checkpatch: Add a --strict test for structs with bool
 member definitions
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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.