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.