USB documentation: explain lifetime rules for unlinking URBs
This patch (as1534c) updates the documentation for usb_unlink_urb and
related functions. It explains that the caller must prevent the URB
being unlinked from getting deallocated while the unlink is taking
place.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/Documentation/usb/URB.txt b/Documentation/usb/URB.txt
index 8ffce74..00d2c64 100644
--- a/Documentation/usb/URB.txt
+++ b/Documentation/usb/URB.txt
@@ -168,6 +168,28 @@
they will get a -EPERM error. Thus you can be sure that when
usb_kill_urb() returns, the URB is totally idle.
+There is a lifetime issue to consider. An URB may complete at any
+time, and the completion handler may free the URB. If this happens
+while usb_unlink_urb or usb_kill_urb is running, it will cause a
+memory-access violation. The driver is responsible for avoiding this,
+which often means some sort of lock will be needed to prevent the URB
+from being deallocated while it is still in use.
+
+On the other hand, since usb_unlink_urb may end up calling the
+completion handler, the handler must not take any lock that is held
+when usb_unlink_urb is invoked. The general solution to this problem
+is to increment the URB's reference count while holding the lock, then
+drop the lock and call usb_unlink_urb or usb_kill_urb, and then
+decrement the URB's reference count. You increment the reference
+count by calling
+
+ struct urb *usb_get_urb(struct urb *urb)
+
+(ignore the return value; it is the same as the argument) and
+decrement the reference count by calling usb_free_urb. Of course,
+none of this is necessary if there's no danger of the URB being freed
+by the completion handler.
+
1.7. What about the completion handler?