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
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;
> 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:
_Boll *ptr = &var;
Which is something that comes apart with bitfields.