On Thu, Mar 29, 2007 at 01:59:58PM +0200, Oliver Neukum wrote:
> Hi,
>
> another one in my traversal through the serial drivers.
>
> This here is beautiful in a haiku sort of way.
> You must read the urb's status before you declare it non busy or
> you may read the next urb's status, not your own, and you must guard
> against write reordering.
>
> Regards
> Oliver
> Signed-off-by: Oliver Neukum <[EMAIL PROTECTED]>
> --
> --- a/drivers/usb/serial/ipw.c 2007-03-29 13:50:52.000000000 +0200
> +++ b/drivers/usb/serial/ipw.c 2007-03-29 13:53:46.000000000 +0200
> @@ -369,13 +369,16 @@
> static void ipw_write_bulk_callback(struct urb *urb)
> {
> struct usb_serial_port *port = urb->context;
> + int status;
>
> dbg("%s", __FUNCTION__);
>
> + status = urb->status;
> + smp_mb(); /* status must be read before urb is declared unbusy */
Why is this necessary? 'status' should always be safe to read while
within the urb callback. If not, then we need to add this to _every_ urb
callback?
Why is this necessary? How can that code be "wrong"?
thanks,
greg k-h
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