locking, sched: Give waitqueue spinlocks their own lockdep classes

Give waitqueue spinlocks their own lockdep classes when they
are initialised from init_waitqueue_head(). This means that
struct wait_queue::func functions can operate other waitqueues.

This is used by CacheFiles to catch the page from a backing fs
being unlocked and to wake up another thread to take a copy of
it.

Signed-off-by: Peter Zijlstra <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
Tested-by: Takashi Iwai <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
LKML-Reference: <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

include/linux/wait.h

···
 #define __WAIT_BIT_KEY_INITIALIZER(word, bit)				\
 	{ .flags = word, .bit_nr = bit, }
 
-extern void init_waitqueue_head(wait_queue_head_t *q);
+extern void __init_waitqueue_head(wait_queue_head_t *q, struct lock_class_key *);
+
+#define init_waitqueue_head(q)				\
+	do {						\
+		static struct lock_class_key __key;	\
+							\
+		__init_waitqueue_head((q), &__key);	\
+	} while (0)
 
 #ifdef CONFIG_LOCKDEP
 # define __WAIT_QUEUE_HEAD_INIT_ONSTACK(name) \

kernel/wait.c

···
 #include <linux/wait.h>
 #include <linux/hash.h>
 
-void init_waitqueue_head(wait_queue_head_t *q)
+void __init_waitqueue_head(wait_queue_head_t *q, struct lock_class_key *key)
 {
 	spin_lock_init(&q->lock);
+	lockdep_set_class(&q->lock, key);
 	INIT_LIST_HEAD(&q->task_list);
 }
 
-EXPORT_SYMBOL(init_waitqueue_head);
+EXPORT_SYMBOL(__init_waitqueue_head);
 
 void add_wait_queue(wait_queue_head_t *q, wait_queue_t *wait)
 {