diff options
| -rw-r--r-- | Documentation/usb/URB.txt | 22 | ||||
| -rw-r--r-- | drivers/usb/core/urb.c | 12 |
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(). |