[PATCH] waitqueue: fix clang -Wuninitialized warnings

[Arnd Bergmann <arnd@arndb.de>]

When CONFIG_LOCKDEP is set, every use of DECLARE_WAIT_QUEUE_HEAD_ONSTACK()
produces an annoying 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; })
                                       ^~~~

After playing with it for a while, I have found a way to rephrase the
macro in a way that should work well with both gcc and clang and not
produce this warning. The open-coded __WAIT_QUEUE_HEAD_INIT_ONSTACK
is a little more verbose than the original version by Peter Zijlstra,
but avoids the gcc-ism that suppresses warnings when assigning a
variable to itself.

Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 include/linux/wait.h | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/linux/wait.h b/include/linux/wait.h
index ddb959641709..84e39643b780 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -68,8 +68,15 @@ extern void __init_waitqueue_head(struct wait_queue_head *wq_head, const char *n
 	} while (0)
 
 #ifdef CONFIG_LOCKDEP
-# define __WAIT_QUEUE_HEAD_INIT_ONSTACK(name) \
-	({ init_waitqueue_head(&name); name; })
+# define __WAIT_QUEUE_HEAD_INIT_ONSTACK(name) {					\
+	.lock		= __SPIN_LOCK_UNLOCKED(name.lock),			\
+	.head		= ({							\
+		static struct lock_class_key __key;				\
+		lockdep_set_class_and_name(&(name).lock, &__key, # name);	\
+		(struct list_head){ &(name).head, &(name).head };		\
+	}),									\
+}
+
 # define DECLARE_WAIT_QUEUE_HEAD_ONSTACK(name) \
 	struct wait_queue_head name = __WAIT_QUEUE_HEAD_INIT_ONSTACK(name)
 #else
-- 
2.20.0