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

[Arnd Bergmann <arnd@arndb.de>]

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
 #else
 # define DECLARE_WAIT_QUEUE_HEAD_ONSTACK(name) DECLARE_WAIT_QUEUE_HEAD(name)
 #endif
-- 
2.20.0