USB: CDC WDM driver doesn't support non-blocking reads

support for O_NONBLOCK in read and write path
by simply not waiting for data in read or availability
of the write urb in write but returning -EAGAIN
Signed-off-by: Oliver Neukum <oliver@neukum.org>
Tested-by: Marcel Holtmann <marcel@holtmann.org>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/class/cdc-wdm.c

@@ -313,8 +313,13 @@ static ssize_t wdm_write
 	r = usb_autopm_get_interface(desc->intf);
 	if (r < 0)
 		goto outnp;
+
+	if (!file->f_flags && O_NONBLOCK)
 		r = wait_event_interruptible(desc->wait, !test_bit(WDM_IN_USE,
 								&desc->flags));
+	else
+		if (test_bit(WDM_IN_USE, &desc->flags))
+			r = -EAGAIN;
 	if (r < 0)
 		goto out;
 
@@ -377,7 +382,7 @@ outnl:
 static ssize_t wdm_read
 (struct file *file, char __user *buffer, size_t count, loff_t *ppos)
 {
-	int rv, cntr;
+	int rv, cntr = 0;
 	int i = 0;
 	struct wdm_device *desc = file->private_data;
 
@@ -389,10 +394,23 @@ static ssize_t wdm_read
 	if (desc->length == 0) {
 		desc->read = 0;
 retry:
+		if (test_bit(WDM_DISCONNECTING, &desc->flags)) {
+			rv = -ENODEV;
+			goto err;
+		}
 		i++;
+		if (file->f_flags & O_NONBLOCK) {
+			if (!test_bit(WDM_READ, &desc->flags)) {
+				rv = cntr ? cntr : -EAGAIN;
+				goto err;
+			}
+			rv = 0;
+		} else {
 			rv = wait_event_interruptible(desc->wait,
 				test_bit(WDM_READ, &desc->flags));
+		}
 
+		/* may have happened while we slept */
 		if (test_bit(WDM_DISCONNECTING, &desc->flags)) {
 			rv = -ENODEV;
 			goto err;
@@ -448,7 +466,7 @@ retry:
 
 err:
 	mutex_unlock(&desc->rlock);
-	if (rv < 0)
+	if (rv < 0 && rv != -EAGAIN)
 		dev_err(&desc->intf->dev, "wdm_read: exit error\n");
 	return rv;
 }