On Mon, Apr 09, 2012 at 03:36:08PM -0400, John Baldwin wrote: > On Monday, April 09, 2012 3:10:00 pm Konstantin Belousov wrote: > > On Mon, Apr 09, 2012 at 11:01:03AM -0400, John Baldwin wrote: > > > On Saturday, April 07, 2012 11:08:41 pm Warner Losh wrote: > > > > > > > > On Apr 7, 2012, at 8:46 AM, 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. > > > > > > > > Also, more generally, don't create the dev nodes before you are ready to > > > deal with requests. Why do we need to uglify everything here? If you can't > > > do that, you can check a bit in the softc and return EBUSY or ENXIO on open if > > > that bit says that your driver isn't ready to accept requests. > > > > > > I agree, this dosen't actually fix anything as the decision for what to put > > > in your foo_attach() method rather than foo_after_attach() is non-obvious and > > > very arbitrary. > > > > > > The actual bug appears to only be with using 'device_busy()'. I think > > > this should be fixed by making device_busy() better, not by adding > > > this type of obfuscation to attach. It should be trivial to make > > > device_busy() safe to use on a device that is currently being attached > > > which will not require any changes to drivers. > > > > Could you, please, elaborate your proposal ? How do you think device_busy() > > can be enchanced ? > > > > Obvious idea to sleep inside device_busy() until dev->state becomes != > > DS_ATTACHED is no go, IMO. The issue is that this causes immediate deadlocks > > if device_attach() method needs to call destroy_dev() to rollback. > > I think you could have a DS_ATTACHING state and allow device_busy() to work > for DS_ATTACHING. The idea being that it is a bug for a driver to invoke > device_busy() if it is going to fail attach. You may then need to do a fixup > in device_attach() to promote the state from DS_ATTACHED to DS_BUSY when it > returns if there is a non-zero busy count. This is quite good idea, but it still adds burden to device author, although I agree that this is manageable. A scenario I have in mind now is the following: assume that driver needs to create two devfs nodes, lets name them dri/card0 and dri/forcewake0. Driver would perform two make_dev_p(9) calls, and while creation of dri/card0 succeed, consequent creation of dri/forcewake0 could fail for numerous reasons. Now, the driver needs to ensure that cdesvw->d_open() on dri/card0 would return ENXIO until dri/forcewake0 is created. This can be implemented with flag, indeed. But still somewhat muddy, and probably leads to user-visible errors (I mostly worry about graphical login managers). But for single-node drivers it is indeed a nice solution. > > Something like this: > > Index: kern/subr_bus.c > =================================================================== > --- kern/subr_bus.c (revision 234057) > +++ kern/subr_bus.c (working copy) > _at__at_ -2472,12 +2472,13 _at__at_ > void > device_busy(device_t dev) > { > - if (dev->state < DS_ATTACHED) > + if (dev->state < DS_ATTACHING) > panic("device_busy: called for unattached device"); > if (dev->busy == 0 && dev->parent) > device_busy(dev->parent); > dev->busy++; > - dev->state = DS_BUSY; > + if (dev->state == DS_ATTACHED) > + dev->state = DS_BUSY; > } > > /** > _at__at_ -2486,14 +2487,16 _at__at_ > void > device_unbusy(device_t dev) > { > - if (dev->state != DS_BUSY) > + if (dev->busy != 0 && dev->state != DS_BUSY && > + dev->state != DS_ATTACHING) > panic("device_unbusy: called for non-busy device %s", > device_get_nameunit(dev)); > dev->busy--; > if (dev->busy == 0) { > if (dev->parent) > device_unbusy(dev->parent); > - dev->state = DS_ATTACHED; > + if (dev->state == DS_BUSY) > + dev->state = DS_ATTACHED; > } > } > > _at__at_ -2729,6 +2732,7 _at__at_ > device_sysctl_init(dev); > if (!device_is_quiet(dev)) > device_print_child(dev->parent, dev); > + dev->state = DS_ATTACHING; > if ((error = DEVICE_ATTACH(dev)) != 0) { > printf("device_attach: %s%d attach returned %d\n", > dev->driver->name, dev->unit, error); > _at__at_ -2736,11 +2740,15 _at__at_ > devclass_delete_device(dev->devclass, dev); > (void)device_set_driver(dev, NULL); > device_sysctl_fini(dev); > + KASSERT(dev->busy == 0, ("attach failed but busy")); > dev->state = DS_NOTPRESENT; > return (error); > } > device_sysctl_update(dev); > - dev->state = DS_ATTACHED; > + if (dev->busy) > + dev->state = DS_BUSY; > + else > + dev->state = DS_ATTACHED; > dev->flags &= ~DF_DONENOMATCH; > devadded(dev); > return (0); > Index: sys/bus.h > =================================================================== > --- sys/bus.h (revision 234057) > +++ sys/bus.h (working copy) > _at__at_ -52,6 +52,7 _at__at_ > typedef enum device_state { > DS_NOTPRESENT = 10, /**< _at_brief not probed or probe failed */ > DS_ALIVE = 20, /**< _at_brief probe succeeded */ > + DS_ATTACHING = 25, /**< _at_brief currently attaching */ > DS_ATTACHED = 30, /**< _at_brief attach method called */ > DS_BUSY = 40 /**< _at_brief device is open */ > } device_state_t; > > -- > John Baldwin
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