USB: change locking for device-level autosuspend

This patch (as1323) changes the locking requirements for
usb_autosuspend_device(), usb_autoresume_device(), and
usb_try_autosuspend_device().  This isn't a very important change;
mainly it's meant to make the locking more uniform.

The most tricky part of the patch involves changes to usbdev_open().
To avoid an ABBA locking problem, it was necessary to reduce the
region protected by usbfs_mutex.  Since that mutex now protects only
against simultaneous open and remove, this posed no difficulty -- its
scope was larger than necessary.

And it turns out that usbfs_mutex is no longer needed in
usbdev_release() at all.  The list of usbfs "ps" structures is now
protected by the device lock instead of by usbfs_mutex.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/core/devio.c

@@ -654,19 +654,21 @@ static int usbdev_open(struct inode *inode, struct file *file)
 	int ret;
 
 	lock_kernel();
-	/* Protect against simultaneous removal or release */
-	mutex_lock(&usbfs_mutex);
 
 	ret = -ENOMEM;
 	ps = kmalloc(sizeof(struct dev_state), GFP_KERNEL);
 	if (!ps)
-		goto out;
+		goto out_free_ps;
 
 	ret = -ENODEV;
 
+	/* Protect against simultaneous removal or release */
+	mutex_lock(&usbfs_mutex);
+
 	/* usbdev device-node */
 	if (imajor(inode) == USB_DEVICE_MAJOR)
 		dev = usbdev_lookup_by_devt(inode->i_rdev);
+
 #ifdef CONFIG_USB_DEVICEFS
 	/* procfs file */
 	if (!dev) {
@@ -678,13 +680,19 @@ static int usbdev_open(struct inode *inode, struct file *file)
 			dev = NULL;
 	}
 #endif
-	if (!dev || dev->state == USB_STATE_NOTATTACHED)
-		goto out;
+	mutex_unlock(&usbfs_mutex);
+
+	if (!dev)
+		goto out_free_ps;
+
+	usb_lock_device(dev);
+	if (dev->state == USB_STATE_NOTATTACHED)
+		goto out_unlock_device;
+
 	ret = usb_autoresume_device(dev);
 	if (ret)
-		goto out;
+		goto out_unlock_device;
 
-	ret = 0;
 	ps->dev = dev;
 	ps->file = file;
 	spin_lock_init(&ps->lock);
@@ -702,14 +710,17 @@ static int usbdev_open(struct inode *inode, struct file *file)
 	smp_wmb();
 	list_add_tail(&ps->list, &dev->filelist);
 	file->private_data = ps;
+	usb_unlock_device(dev);
 	snoop(&dev->dev, "opened by process %d: %s\n", task_pid_nr(current),
 			current->comm);
- out:
-	if (ret) {
-		kfree(ps);
+	unlock_kernel();
+	return ret;
+
+ out_unlock_device:
+	usb_unlock_device(dev);
 	usb_put_dev(dev);
-	}
-	mutex_unlock(&usbfs_mutex);
+ out_free_ps:
+	kfree(ps);
 	unlock_kernel();
 	return ret;
 }
@@ -724,10 +735,7 @@ static int usbdev_release(struct inode *inode, struct file *file)
 	usb_lock_device(dev);
 	usb_hub_release_all_ports(dev, ps);
 
-	/* Protect against simultaneous open */
-	mutex_lock(&usbfs_mutex);
 	list_del_init(&ps->list);
-	mutex_unlock(&usbfs_mutex);
 
 	for (ifnum = 0; ps->ifclaimed && ifnum < 8*sizeof(ps->ifclaimed);
 			ifnum++) {

drivers/usb/core/driver.c

@@ -1478,8 +1478,7 @@ void usb_autoresume_work(struct work_struct *work)
  * driver requires remote-wakeup capability during autosuspend but remote
  * wakeup is disabled, the autosuspend will fail.
  *
- * Often the caller will hold @udev's device lock, but this is not
- * necessary.
+ * The caller must hold @udev's device lock.
  *
  * This routine can run only in process context.
  */
@@ -1503,6 +1502,8 @@ void usb_autosuspend_device(struct usb_device *udev)
  * for an active interface is greater than 0, or autosuspend is not allowed
  * for any other reason, no autosuspend request will be queued.
  *
+ * The caller must hold @udev's device lock.
+ *
  * This routine can run only in process context.
  */
 void usb_try_autosuspend_device(struct usb_device *udev)
@@ -1526,8 +1527,7 @@ void usb_try_autosuspend_device(struct usb_device *udev)
  * @udev's usage counter is incremented to prevent subsequent autosuspends.
  * However if the autoresume fails then the usage counter is re-decremented.
  *
- * Often the caller will hold @udev's device lock, but this is not
- * necessary (and attempting it might cause deadlock).
+ * The caller must hold @udev's device lock.
  *
  * This routine can run only in process context.
  */

drivers/usb/core/sysfs.c

@@ -352,6 +352,7 @@ set_autosuspend(struct device *dev, struct device_attribute *attr,
 		return -EINVAL;
 	value *= HZ;
 
+	usb_lock_device(udev);
 	udev->autosuspend_delay = value;
 	if (value >= 0)
 		usb_try_autosuspend_device(udev);
@@ -359,6 +360,7 @@ set_autosuspend(struct device *dev, struct device_attribute *attr,
 		if (usb_autoresume_device(udev) == 0)
 			usb_autosuspend_device(udev);
 	}
+	usb_unlock_device(udev);
 	return count;
 }