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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
parentbcf398537630bf20b4dbe59ba855b69f404c93cf (diff)
downloadlinux-linaro-stable-da8bfb090c2b30af9f3879443355f7eb1d0fe10a.tar.gz
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>
-rw-r--r--Documentation/usb/URB.txt22
-rw-r--r--drivers/usb/core/urb.c12
2 files changed, 34 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?
diff --git a/drivers/usb/core/urb.c b/drivers/usb/core/urb.c
index 7239a73c1b8c..cd9b3a2cd8a7 100644
--- a/drivers/usb/core/urb.c
+++ b/drivers/usb/core/urb.c
@@ -539,6 +539,10 @@ EXPORT_SYMBOL_GPL(usb_submit_urb);
* never submitted, or it was unlinked before, or the hardware is already
* finished with it), even if the completion handler has not yet run.
*
+ * The URB must not be deallocated while this routine is running. In
+ * particular, when a driver calls this routine, it must insure that the
+ * completion handler cannot deallocate the URB.
+ *
* Unlinking and Endpoint Queues:
*
* [The behaviors and guarantees described below do not apply to virtual
@@ -603,6 +607,10 @@ EXPORT_SYMBOL_GPL(usb_unlink_urb);
* with error -EPERM. Thus even if the URB's completion handler always
* tries to resubmit, it will not succeed and the URB will become idle.
*
+ * The URB must not be deallocated while this routine is running. In
+ * particular, when a driver calls this routine, it must insure that the
+ * completion handler cannot deallocate the URB.
+ *
* This routine may not be used in an interrupt context (such as a bottom
* half or a completion handler), or when holding a spinlock, or in other
* situations where the caller can't schedule().
@@ -640,6 +648,10 @@ EXPORT_SYMBOL_GPL(usb_kill_urb);
* with error -EPERM. Thus even if the URB's completion handler always
* tries to resubmit, it will not succeed and the URB will become idle.
*
+ * The URB must not be deallocated while this routine is running. In
+ * particular, when a driver calls this routine, it must insure that the
+ * completion handler cannot deallocate the URB.
+ *
* This routine may not be used in an interrupt context (such as a bottom
* half or a completion handler), or when holding a spinlock, or in other
* situations where the caller can't schedule().