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

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jul 19, 2019 at 01:36:00PM +0200, Arnd Bergmann wrote:
> When CONFIG_LOCKDEP is set, every use of DECLARE_WAIT_QUEUE_HEAD_ONSTACK()
> produces an bogus warning from clang, which is particularly annoying
> for allmodconfig builds:
> 
> fs/namei.c:1646:34: error: variable 'wq' is uninitialized when used within its own initialization [-Werror,-Wuninitialized]
>         DECLARE_WAIT_QUEUE_HEAD_ONSTACK(wq);
>                                         ^~
> include/linux/wait.h:74:63: note: expanded from macro 'DECLARE_WAIT_QUEUE_HEAD_ONSTACK'
>         struct wait_queue_head name = __WAIT_QUEUE_HEAD_INIT_ONSTACK(name)
>                                ~~~~                                  ^~~~
> include/linux/wait.h:72:33: note: expanded from macro '__WAIT_QUEUE_HEAD_INIT_ONSTACK'
>         ({ init_waitqueue_head(&name); name; })
>                                        ^~~~
> 
> A patch for clang has already been proposed and should soon be
> merged for clang-9, but for now all clang versions produce the
> warning in an otherwise (almost) clean allmodconfig build.
> 
> Link: https://bugs.llvm.org/show_bug.cgi?id=31829
> Link: https://bugs.llvm.org/show_bug.cgi?id=42604
> Link: https://lore.kernel.org/lkml/20190703081119.209976-1-arnd@arndb.de/
> Link: https://reviews.llvm.org/D64678
> Link: https://storage.kernelci.org/next/master/next-20190717/arm64/allmodconfig/clang-8/build-warnings.log
> Suggested-by: Nathan Chancellor <natechancellor@gmail.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> v2: given that kernelci is getting close to reporting a clean build for
>     clang, I'm trying again with a less invasive approach after my
>     first version was not too popular.
> ---
>  include/linux/wait.h | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/wait.h b/include/linux/wait.h
> index ddb959641709..276499ae1a3e 100644
> --- 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.