On Tue, 24 Apr 2012, Oliver Neukum wrote: > Am Montag, 23. April 2012, 17:42:11 schrieb Alan Stern: > > I don't like the idea of changing the status codes. It would mean > > changing usb_kill_urb too. > > Why? Okay, maybe it wouldn't. If usb_unlink_urb returned -ENOENT as the status for synchronous givebacks then they could be treated the same as usb_kill_urb's synchronous completions. But it certainly would mean changing every HCD. > > Instead of changing return codes or adding locks, how about > > implementing a small state machine for each URB? > > > > Initially the state is ACTIVE. > > > > When the URB times out, acquire the lock. If the state is not > > equal to ACTIVE, drop the lock and return immediately (the URB > > is being unlinked concurrently). Otherwise set the state to > > UNLINK_STARTED, drop the lock, call usb_unlink_urb, and > > reacquire the lock. If the state hasn't changed, set it back > > to ACTIVE. But if the state has changed to UNLINK_FINISHED, > > set it to ACTIVE and resubmit. > > > > In the completion handler, grab the lock. If the state > > is ACTIVE, resubmit. But if the state is UNLINK_STARTED, > > change it to UNLINK_FINISHED and don't resubmit. > > > > This is a better approach, in that it doesn't make any assumptions > > regarding synchronous vs. asynchronous unlinks. If you want, you could > > have two different ACTIVE substates, one for URBs which haven't yet > > been unlinked and one for URBs which have been. Then you could avoid > > unlinking the same URB twice. > > That would work, provided we extend the status machine by an error > code when resubmission is not desired Presumably there would be a "resubmit" function which could be called by both the completion handler and the unlink handler. It should be able to tell if a resubmission was not desired, with no need for extra state information. How does the current driver decide whether to resubmit? > and check for UNLINK_STARTED > also when a timeout begins. That was part of my description above: "If the state is not equal to ACTIVE, drop the lock and return immediately (the URB is being unlinked concurrently)." > But I wouldn't call that a simple solution. Well, that's a matter of opinion. It does have the advantage, unlike lots of other proposals in this email thread, of being a _correct_ solution. Alan Stern -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
