From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH] checkpatch: Add a --strict test for structs with bool member definitions Date: Wed, 11 Apr 2018 19:00:49 +0200 Message-ID: <20180411170049.GR4082@hirez.programming.kicks-ass.net> References: <891d4f632fbff5052e11f2d0b6fac35d@codeaurora.org> <20180410123305.GF4082@hirez.programming.kicks-ass.net> <95477c93db187bab6da8a8ba7c57836868446179.camel@perches.com> <20180410143950.4b8526073b4e3e34689f68cb@linux-foundation.org> <20180410150011.df9e036f57b5bcac7ac19686@linux-foundation.org> <20180411081502.GJ4082@hirez.programming.kicks-ass.net> <20180411092959.e666ec443e4d3bb6f43901d7@linux-foundation.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20180411092959.e666ec443e4d3bb6f43901d7@linux-foundation.org> Sender: linux-kernel-owner@vger.kernel.org To: Andrew Morton Cc: Joe Perches , "Rafael J. Wysocki" , Andy Whitcroft , yuankuiz@codeaurora.org, Linux PM , "Rafael J. Wysocki" , Frederic Weisbecker , Thomas Gleixner , paulmck@linux.vnet.ibm.com, Ingo Molnar , Len Brown , Linux Kernel Mailing List List-Id: linux-pm@vger.kernel.org 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 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.