userfaultfd: non-cooperative: Split the find_userfault() routine

I will need one to lookup for userfaultfd_wait_queue-s in different
wait queue

Link: http://lkml.kernel.org/r/20161216144821.5183-6-aarcange@redhat.com
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Michael Rapoport <RAPOPORT@il.ibm.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/userfaultfd.c

@@ -522,25 +522,30 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
 }
 
 /* fault_pending_wqh.lock must be hold by the caller */
-static inline struct userfaultfd_wait_queue *find_userfault(
-	struct userfaultfd_ctx *ctx)
+static inline struct userfaultfd_wait_queue *find_userfault_in(
+		wait_queue_head_t *wqh)
 {
 	wait_queue_t *wq;
 	struct userfaultfd_wait_queue *uwq;
 
-	VM_BUG_ON(!spin_is_locked(&ctx->fault_pending_wqh.lock));
+	VM_BUG_ON(!spin_is_locked(&wqh->lock));
 
 	uwq = NULL;
-	if (!waitqueue_active(&ctx->fault_pending_wqh))
+	if (!waitqueue_active(wqh))
 		goto out;
 	/* walk in reverse to provide FIFO behavior to read userfaults */
-	wq = list_last_entry(&ctx->fault_pending_wqh.task_list,
-			     typeof(*wq), task_list);
+	wq = list_last_entry(&wqh->task_list, typeof(*wq), task_list);
 	uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
 out:
 	return uwq;
 }
 
+static inline struct userfaultfd_wait_queue *find_userfault(
+		struct userfaultfd_ctx *ctx)
+{
+	return find_userfault_in(&ctx->fault_pending_wqh);
+}
+
 static unsigned int userfaultfd_poll(struct file *file, poll_table *wait)
 {
 	struct userfaultfd_ctx *ctx = file->private_data;