sched/headers: fix up header file dependency on <linux/sched/signal.h>

The scheduler header file split and cleanups ended up exposing a few
nasty header file dependencies, and in particular it showed how we in
<linux/wait.h> ended up depending on "signal_pending()", which now comes
from <linux/sched/signal.h>.

That's a very subtle and annoying dependency, which already caused a
semantic merge conflict (see commit e58bc927 "Pull overlayfs updates
from Miklos Szeredi", which added that fixup in the merge commit).

It turns out that we can avoid this dependency _and_ improve code
generation by moving the guts of the fairly nasty helper #define
__wait_event_interruptible_locked() to out-of-line code.  The code that
includes the signal_pending() check is all in the slow-path where we
actually go to sleep waiting for the event anyway, so using a helper
function is the right thing to do.

Using a helper function is also what we already did for the non-locked
versions, see the "__wait_event*()" macros and the "prepare_to_wait*()"
set of helper functions.

We might want to try to unify all these macro games, we have a _lot_ of
subtly different wait-event loops.  But this is the minimal patch to fix
the annoying header dependency.
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/wait.h

@@ -620,30 +620,19 @@ do {									\
 	__ret;								\
 })
 
+extern int do_wait_intr(wait_queue_head_t *, wait_queue_t *);
+extern int do_wait_intr_irq(wait_queue_head_t *, wait_queue_t *);
 
-#define __wait_event_interruptible_locked(wq, condition, exclusive, irq) \
+#define __wait_event_interruptible_locked(wq, condition, exclusive, fn) \
 ({									\
-	int __ret = 0;							\
+	int __ret;							\
 	DEFINE_WAIT(__wait);						\
 	if (exclusive)							\
 		__wait.flags |= WQ_FLAG_EXCLUSIVE;			\
 	do {								\
-		if (likely(list_empty(&__wait.task_list)))		\
-			__add_wait_queue_tail(&(wq), &__wait);		\
-		set_current_state(TASK_INTERRUPTIBLE);			\
-		if (signal_pending(current)) {				\
-			__ret = -ERESTARTSYS;				\
+		__ret = fn(&(wq), &__wait);				\
+		if (__ret)						\
 			break;						\
-		}							\
-		if (irq)						\
-			spin_unlock_irq(&(wq).lock);			\
-		else							\
-			spin_unlock(&(wq).lock);			\
-		schedule();						\
-		if (irq)						\
-			spin_lock_irq(&(wq).lock);			\
-		else							\
-			spin_lock(&(wq).lock);				\
 	} while (!(condition));						\
 	__remove_wait_queue(&(wq), &__wait);				\
 	__set_current_state(TASK_RUNNING);				\
@@ -676,7 +665,7 @@ do {									\
  */
 #define wait_event_interruptible_locked(wq, condition)			\
 	((condition)							\
-	 ? 0 : __wait_event_interruptible_locked(wq, condition, 0, 0))
+	 ? 0 : __wait_event_interruptible_locked(wq, condition, 0, do_wait_intr))
 
 /**
  * wait_event_interruptible_locked_irq - sleep until a condition gets true
@@ -703,7 +692,7 @@ do {									\
  */
 #define wait_event_interruptible_locked_irq(wq, condition)		\
 	((condition)							\
-	 ? 0 : __wait_event_interruptible_locked(wq, condition, 0, 1))
+	 ? 0 : __wait_event_interruptible_locked(wq, condition, 0, do_wait_intr_irq))
 
 /**
  * wait_event_interruptible_exclusive_locked - sleep exclusively until a condition gets true
@@ -734,7 +723,7 @@ do {									\
  */
 #define wait_event_interruptible_exclusive_locked(wq, condition)	\
 	((condition)							\
-	 ? 0 : __wait_event_interruptible_locked(wq, condition, 1, 0))
+	 ? 0 : __wait_event_interruptible_locked(wq, condition, 1, do_wait_intr))
 
 /**
  * wait_event_interruptible_exclusive_locked_irq - sleep until a condition gets true
@@ -765,7 +754,7 @@ do {									\
  */
 #define wait_event_interruptible_exclusive_locked_irq(wq, condition)	\
 	((condition)							\
-	 ? 0 : __wait_event_interruptible_locked(wq, condition, 1, 1))
+	 ? 0 : __wait_event_interruptible_locked(wq, condition, 1, do_wait_intr_irq))
 
 
 #define __wait_event_killable(wq, condition)				\

kernel/sched/wait.c

@@ -242,6 +242,45 @@ long prepare_to_wait_event(wait_queue_head_t *q, wait_queue_t *wait, int state)
 }
 EXPORT_SYMBOL(prepare_to_wait_event);
 
+/*
+ * Note! These two wait functions are entered with the
+ * wait-queue lock held (and interrupts off in the _irq
+ * case), so there is no race with testing the wakeup
+ * condition in the caller before they add the wait
+ * entry to the wake queue.
+ */
+int do_wait_intr(wait_queue_head_t *wq, wait_queue_t *wait)
+{
+	if (likely(list_empty(&wait->task_list)))
+		__add_wait_queue_tail(wq, wait);
+
+	set_current_state(TASK_INTERRUPTIBLE);
+	if (signal_pending(current))
+		return -ERESTARTSYS;
+
+	spin_unlock(&wq->lock);
+	schedule();
+	spin_lock(&wq->lock);
+	return 0;
+}
+EXPORT_SYMBOL(do_wait_intr);
+
+int do_wait_intr_irq(wait_queue_head_t *wq, wait_queue_t *wait)
+{
+	if (likely(list_empty(&wait->task_list)))
+		__add_wait_queue_tail(wq, wait);
+
+	set_current_state(TASK_INTERRUPTIBLE);
+	if (signal_pending(current))
+		return -ERESTARTSYS;
+
+	spin_unlock_irq(&wq->lock);
+	schedule();
+	spin_lock_irq(&wq->lock);
+	return 0;
+}
+EXPORT_SYMBOL(do_wait_intr_irq);
+
 /**
  * finish_wait - clean up after waiting in a queue
  * @q: waitqueue waited on