HID: update BKL comment in hiddev

Update comment explaining BKL usage in legacy hiddev driver.

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

1 files changed, 17 insertions, 2 deletions

diff --git a/drivers/hid/usbhid/hiddev.c b/drivers/hid/usbhid/hiddev.c
index 433602aed468..c24d2fa3e3b6 100644
--- 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) {