Re: device_attach(9) and driver initialization

On Sat, Apr 07, 2012 at 09:10:55PM -0600, Warner Losh wrote:
> 
> On Apr 7, 2012, at 8:57 AM, Konstantin Belousov wrote:
> 
> > On Sat, Apr 07, 2012 at 08:46:41AM -0600, Ian Lepore wrote:
> >> On Sat, 2012-04-07 at 15:50 +0300, Konstantin Belousov wrote:
> >>> Hello,
> >>> there seems to be a problem with device attach sequence offered by newbus.
> >>> Basically, when device attach method is executing, device is not fully
> >>> initialized yet. Also the device state in the newbus part of the world
> >>> is DS_ALIVE. There is definitely no shattering news in the statements,
> >>> but drivers that e.g. create devfs node to communicate with consumers
> >>> are prone to a race.
> >>> 
> >>> If /dev node is created inside device attach method, then usermode
> >>> can start calling cdevsw methods before device fully initialized itself.
> >>> Even more, if device tries to use newbus helpers in cdevsw methods,
> >>> like device_busy(9), then panic occurs "called for unatteched device".
> >>> I get reports from users about this issues, to it is not something
> >>> that only could happen.
> >>> 
> >>> I propose to add DEVICE_AFTER_ATTACH() driver method, to be called
> >>> from newbus right after device attach finished and newbus considers
> >>> the device fully initialized. Driver then could create devfs node
> >>> in the after_attach method instead of attach. Please see the patch below.
> >>> 
> >>> diff --git a/sys/kern/device_if.m b/sys/kern/device_if.m
> >>> index eb720eb..9db74e2 100644
> >>> --- a/sys/kern/device_if.m
> >>> +++ b/sys/kern/device_if.m
> >>> _at__at_ -43,6 +43,10 _at__at_ INTERFACE device;
> >>> # Default implementations of some methods.
> >>> #
> >>> CODE {
> >>> +	static void null_after_attach(device_t dev)
> >>> +	{
> >>> +	}
> >>> +
> >>> 	static int null_shutdown(device_t dev)
> >>> 	{
> >>> 	    return 0;
> >>> _at__at_ -199,6 +203,21 _at__at_ METHOD int attach {
> >>> };
> >>> 
> >>> /**
> >>> + * _at_brief Notify the driver that device is in attached state
> >>> + *
> >>> + * Called after driver is successfully attached to the device and
> >>> + * corresponding device_t is fully operational. Driver now may expose
> >>> + * the device to the consumers, e.g. create devfs nodes.
> >>> + *
> >>> + * _at_param dev		the device to probe
> >>> + *
> >>> + * _at_see DEVICE_ATTACH()
> >>> + */
> >>> +METHOD void after_attach {
> >>> +	device_t dev;
> >>> +} DEFAULT null_after_attach;
> >>> +
> >>> +/**
> >>>  * _at_brief Detach a driver from a device.
> >>>  *
> >>>  * This can be called if the user is replacing the
> >>> diff --git a/sys/kern/subr_bus.c b/sys/kern/subr_bus.c
> >>> index d485b9f..6d849cb 100644
> >>> --- a/sys/kern/subr_bus.c
> >>> +++ b/sys/kern/subr_bus.c
> >>> _at__at_ -2743,6 +2743,7 _at__at_ device_attach(device_t dev)
> >>> 	dev->state = DS_ATTACHED;
> >>> 	dev->flags &= ~DF_DONENOMATCH;
> >>> 	devadded(dev);
> >>> +	DEVICE_AFTER_ATTACH(dev);
> >>> 	return (0);
> >>> }
> >>> 
> >> 
> >> Does device_get_softc() work before attach is completed?  (I don't have
> >> time to go look in the code right now).  If so, then a mutex initialized
> >> and acquired early in the driver's attach routine, and also acquired in
> >> the driver's cdev implementation routines before using any newbus
> >> functions other than device_get_softc(), would solve the problem without
> >> a driver api change that would make it harder to backport/MFC driver
> >> changes.
> > No, 'a mutex' does not solve anything. It only adds enourmous burden
> > on the driver developers, because you cannot sleep under mutex. Changing
> > the mutex to the sleepable lock also does not byy you much, since
> > you need to somehow solve the issues with some cdevsw call waking up
> > thread sleeping into another cdevsw call, just for example.
> > 
> > Singlethreading a driver due to this race is just silly.
> > 
> > And, what do you mean by 'making it harder to MFC' ? How ?
> 
> driver_attach()
> {
> 	...
> 	softc->flags = 0; // redundant, since softc is initialized to 0.
> 	softc->cdev = device_create...();
> 	...
> 	softc->flags |= READY;
> }
> 
> driver_open(...)
> {
> 	if (!(softc->flags & READY))
> 		return ENXIO;
> 	...
> }
> 
> What's the big burden here?
The burden is that your proposal does not work. As I described above,
device_busy() calls from cdevsw method panic if open() is called before
DS_ATTACHED is set by newbus. And, DS_ATTACHED is only set after device
attach() method returned, so no workarounds from attach() could solve
this.