path: root/Documentation/usb
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authorAlan Stern <stern@rowland.harvard.edu>2012-03-28 16:13:28 -0400
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2012-04-06 13:54:00 -0700
commitda8bfb090c2b30af9f3879443355f7eb1d0fe10a (patch)
tree377d67c3c0cc1ad40d7357f821712c4c9ba6c639 /Documentation/usb
parentbcf398537630bf20b4dbe59ba855b69f404c93cf (diff)
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>
Diffstat (limited to 'Documentation/usb')
1 files changed, 22 insertions, 0 deletions
diff --git a/Documentation/usb/URB.txt b/Documentation/usb/URB.txt
index 8ffce746d496..00d2c644068e 100644
--- a/Documentation/usb/URB.txt
+++ b/Documentation/usb/URB.txt
@@ -168,6 +168,28 @@ that if the completion handler or anyone else tries to resubmit it
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?

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