Re: [PATCH]generic usb serial driver

On Thursday 29 March 2007 11:58 pm, Oliver Neukum wrote:

> 	/* complete() can reenter this HCD */
> 	spin_unlock (&ehci->lock);
> 	usb_hcd_giveback_urb (ehci_to_hcd(ehci), urb);
> 	spin_lock (&ehci->lock);
> 
> The comment gives you the reason and is right. The locks must be dropped.
> Therefore, if the urb is resubmitted while the completion handler is running,
> usbcore will fully process this urb.

Well, "fully" as in "hand it back to the HCD" (it's effectively a new URB).


> This means that the status will be set to -EINPROGRESS.
> So the status is valid only until the urb is resubmitted. The driver
> in question may resubmit the urb in the write path if !port->write_urb_busy.
> 
> We must consider clearing a busy flag equivalent to resubmitting the
> urb ourselves. 

Sounds like the issue would be a locking bug with respect to that
driver-internal flag ...


> In a completion handler that resubmits the urb you do that 
> resubmission last in the handler, don't you?

And before then, urb->status won't change, since nobody else is
using that URB.  If not, that's clearly a locking bug...


> As an added complication, the resubmission would happen on another cpu.
> Therefore the order on the bus must be correct, hence the "smp_mb()".

Erm, resubmit would happen on the CPU running the completion handler.
So, no mb() or even smp_mb() is required for resubmit-from-compltion.

- Dave

-------------------------------------------------------------------------
Take Surveys. Earn Cash. Influence the Future of IT
Join SourceForge.net's Techsay panel and you'll get the chance to share your
opinions on IT & business topics through brief surveys-and earn cash
http://www.techsay.com/default.php?page=join.php&p=sourceforge&CID=DEVDEV
_______________________________________________
[email protected]
To unsubscribe, use the last form field at:
https://lists.sourceforge.net/lists/listinfo/linux-usb-devel