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

Am Mittwoch, 11. Juli 2007 schrieb jidong xiao:
> 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?

The handler is called with a spinlock held. If you sleep and another
task tries to take the same lock, you will get a deadlock. This is the
general reason for being unable to sleep in USB drivers.

	Regards
		Oliver

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