Re: What's the difference between async and sync unlink?

On 7/11/07, Oliver Neukum <[email protected]> wrote:
> Am Mittwoch, 11. Juli 2007 schrieb jidong xiao:
> > I see there are two routines, usb_unlink_urb and usb_kill_urb,the
> > latter one should be used for sync unlinking,this means it can go to
> > sleep,it won't return until the completion handler have finished.And
> > the former one could not go to sleep.But what's difference between
> > sync and async,I mean,why do we need such two kinds of mechanism to
> > cancel a transfer request?
>
> Under some conditions, eg. network drivers' and the storage driver's
> timeout handling, drivers cannot sleep. They have to stop transfers
> asynchronously.

Ok,let's talk about the timeout handling routine,we defined a
timeout_handler in drivers/usb/storage/transport.c,inside that
function,we will call usb_unlink_urb,why cannot we sleep?what impact
if the driver go to sleep here?

/* This is the timeout handler which will cancel an URB when its timeout
 * expires.
 */
static void timeout_handler(unsigned long us_)
{
	struct us_data *us = (struct us_data *) us_;

	if (test_and_clear_bit(US_FLIDX_URB_ACTIVE, &us->flags)) {
		US_DEBUGP("Timeout -- cancelling URB\n");
		usb_unlink_urb(us->current_urb);
	}
}

Regards
Jason

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