fs: introduce new ->get_poll_head and ->poll_mask methods

->get_poll_head returns the waitqueue that the poll operation is going
to sleep on.  Note that this means we can only use a single waitqueue
for the poll, unlike some current drivers that use two waitqueues for
different events.  But now that we have keyed wakeups and heavily use
those for poll there aren't that many good reason left to keep the
multiple waitqueues, and if there are any ->poll is still around, the
driver just won't support aio poll.

Signed-off-by: Christoph Hellwig <[email protected]>
Reviewed-by: Greg Kroah-Hartman <[email protected]>
Reviewed-by: Darrick J. Wong <[email protected]>

fs/select.c

@@ -34,6 +34,29 @@
 
 #include <linux/uaccess.h>
 
+__poll_t vfs_poll(struct file *file, struct poll_table_struct *pt)
+{
+	if (file->f_op->poll) {
+		return file->f_op->poll(file, pt);
+	} else if (file_has_poll_mask(file)) {
+		unsigned int events = poll_requested_events(pt);
+		struct wait_queue_head *head;
+
+		if (pt && pt->_qproc) {
+			head = file->f_op->get_poll_head(file, events);
+			if (!head)
+				return DEFAULT_POLLMASK;
+			if (IS_ERR(head))
+				return EPOLLERR;
+			pt->_qproc(file, head, pt);
+		}
+
+		return file->f_op->poll_mask(file, events);
+	} else {
+		return DEFAULT_POLLMASK;
+	}
+}
+EXPORT_SYMBOL_GPL(vfs_poll);
 
 /*
  * Estimate expected accuracy in ns from a timeval.