Re: [PATCH] [v2] waitqueue: shut up clang -Wuninitialized warnings

[Nathan Chancellor <natechancellor@gmail.com>]

On Tue, Jul 23, 2019 at 01:03:05PM +0200, Arnd Bergmann wrote:
> On Tue, Jul 23, 2019 at 12:50 PM Peter Zijlstra <peterz@infradead.org> wrote:
> > On Fri, Jul 19, 2019 at 01:36:00PM +0200, Arnd Bergmann wrote:
> > > --- a/include/linux/wait.h
> > > +++ b/include/linux/wait.h
> > > @@ -70,8 +70,17 @@ extern void __init_waitqueue_head(struct wait_queue_head *wq_head, const char *n
> > >  #ifdef CONFIG_LOCKDEP
> > >  # define __WAIT_QUEUE_HEAD_INIT_ONSTACK(name) \
> > >       ({ init_waitqueue_head(&name); name; })
> > > -# define DECLARE_WAIT_QUEUE_HEAD_ONSTACK(name) \
> > > +# if defined(__clang__) && __clang_major__ <= 9
> > > +/* work around https://bugs.llvm.org/show_bug.cgi?id=42604 */
> > > +#  define DECLARE_WAIT_QUEUE_HEAD_ONSTACK(name)                                      \
> > > +     _Pragma("clang diagnostic push")                                        \
> > > +     _Pragma("clang diagnostic ignored \"-Wuninitialized\"")                 \
> > > +     struct wait_queue_head name = __WAIT_QUEUE_HEAD_INIT_ONSTACK(name)      \
> > > +     _Pragma("clang diagnostic pop")
> > > +# else
> > > +#  define DECLARE_WAIT_QUEUE_HEAD_ONSTACK(name) \
> > >       struct wait_queue_head name = __WAIT_QUEUE_HEAD_INIT_ONSTACK(name)
> > > +# endif
> >
> > While this is indeed much better than before; do we really want to do
> > this? That is, since clang-9 release will not need this, we're basically
> > doing the above for pre-release compilers only.
> 
> Kernelci currently builds arch/arm and arch/arm64 kernels with clang-8,
> and probably won't change to clang-9 until after that is released,
> presumably in September.
> 
> Anyone doing x86 builds would use a clang-9 snapshot today
> because of the asm-goto support, but so far the fix has not
> been merged there either. I think the chances of it getting
> fixed before the release are fairly good, but I don't know how
> long it will actually take.
> 
>        Arnd

Furthermore, while x86 will only be supported by clang-9 and up, there
are other architectures/configurations that work with earlier versions
that will never see that fix. There are a few people that still use
clang-7 for example.

In an ideal world, everyone should be using the latest version of clang
because of all of the fixes and improvements that are going into that
latest version but the same can be said of any piece of software. I am
not sure that it is fair to force someone to upgrade when it works for
them. Not everyone runs Ubuntu/Debian to get access to apt.llvm.org
builds or wants to add random repositories to their list or wants to
build clang from source.

I suppose it comes down to policy: if we don't want to support versions
of LLVM before 9.x then we should just break the build when it is
detected but Nick has spoken out against that and I think that he has a
fair point.

https://lore.kernel.org/lkml/CAKwvOdnzrMOCo4RRsfcR=K5ELWU8obgMqtOGZnx_avLrArjpRQ@mail.gmail.com/

Cheers,
Nathan