HID: update BKL comment in hiddev

Update comment explaining BKL usage in legacy hiddev driver.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>

diff --git a/drivers/hid/usbhid/hiddev.c b/drivers/hid/usbhid/hiddev.c
index 433602a..c24d2fa 100644 (file)
--- a/drivers/hid/usbhid/hiddev.c
+++ b/drivers/hid/usbhid/hiddev.c
@@ -267,6 +267,7 @@ static int hiddev_open(struct inode *inode, struct file *file)
        struct hiddev_list *list;
        int res, i;
 
+       /* See comment in hiddev_connect() for BKL explanation */
        lock_kernel();
        i = iminor(inode) - HIDDEV_MINOR_BASE;
 
@@ -894,8 +895,22 @@ int hiddev_connect(struct hid_device *hid, unsigned int force)
        hiddev->hid = hid;
        hiddev->exist = 1;
 
-       /* when lock_kernel() usage is fixed in usb_open(),
-        * we could also fix it here */
+       /*
+        * BKL here is used to avoid race after usb_register_dev().
+        * Once the device node has been created, open() could happen on it.
+        * The code below will then fail, as hiddev_table hasn't been
+        * updated.
+        *
+        * The obvious fix -- introducing mutex to guard hiddev_table[]
+        * doesn't work, as usb_open() and usb_register_dev() both take
+        * minor_rwsem, thus we'll have ABBA deadlock.
+        *
+        * Before BKL pushdown, usb_open() had been acquiring it in right
+        * order, so _open() was safe to use it to protect from this race.
+        * Now the order is different, but AB-BA deadlock still doesn't occur
+        * as BKL is dropped on schedule() (i.e. while sleeping on
+        * minor_rwsem). Fugly.
+        */
        lock_kernel();
        retval = usb_register_dev(usbhid->intf, &hiddev_class);
        if (retval) {