Date: Tue, 10 Apr 2012 17:38:35 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: John Baldwin <jhb@freebsd.org> Cc: Ian Lepore <freebsd@damnhippie.dyndns.org>, freebsd-drivers@freebsd.org, current@freebsd.org Subject: Re: device_attach(9) and driver initialization Message-ID: <20120410143835.GZ2358@deviant.kiev.zoral.com.ua> In-Reply-To: <201204100956.06750.jhb@freebsd.org> References: <20120407125056.GS2358@deviant.kiev.zoral.com.ua> <201204091536.08399.jhb@freebsd.org> <20120409200529.GT2358@deviant.kiev.zoral.com.ua> <201204100956.06750.jhb@freebsd.org>
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On Tue, Apr 10, 2012 at 09:56:06AM -0400, John Baldwin wrote:
> On Monday, April 09, 2012 4:05:29 pm Konstantin Belousov wrote:
> > 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:
> > > > > >=20
> > > > > > On Apr 7, 2012, at 8:46 AM, Ian Lepore wrote:
> > > > > >=20
> > > > > > > On Sat, 2012-04-07 at 15:50 +0300, Konstantin Belousov wrote:
> > > > > > >> Hello,
> > > > > > >> there seems to be a problem with device attach sequence offe=
red by=20
> > > > > 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 s=
tatements,
> > > > > > >> but drivers that e.g. create devfs node to communicate with =
consumers
> > > > > > >> are prone to a race.
> > > > > > >>=20
> > > > > > >> If /dev node is created inside device attach method, then us=
ermode
> > > > > > >> can start calling cdevsw methods before device fully initial=
ized itself.
> > > > > > >> Even more, if device tries to use newbus helpers in cdevsw m=
ethods,
> > > > > > >> like device_busy(9), then panic occurs "called for unatteche=
d device".
> > > > > > >> I get reports from users about this issues, to it is not som=
ething
> > > > > > >> that only could happen.
> > > > > > >>=20
> > > > > > >> I propose to add DEVICE_AFTER_ATTACH() driver method, to be =
called
> > > > > > >> from newbus right after device attach finished and newbus co=
nsiders
> > > > > > >> the device fully initialized. Driver then could create devfs=
node
> > > > > > >> in the after_attach method instead of attach. Please see the=
patch below.
> > > > > > >>=20
> > > > > > >> 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
> > > > > > >> @@ -43,6 +43,10 @@ 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;
> > > > > > >> @@ -199,6 +203,21 @@ METHOD int attach {
> > > > > > >> };
> > > > > > >>=20
> > > > > > >> /**
> > > > > > >> + * @brief Notify the driver that device is in attached state
> > > > > > >> + *
> > > > > > >> + * Called after driver is successfully attached to the devi=
ce and
> > > > > > >> + * corresponding device_t is fully operational. Driver now =
may expose
> > > > > > >> + * the device to the consumers, e.g. create devfs nodes.
> > > > > > >> + *
> > > > > > >> + * @param dev the device to probe
> > > > > > >> + *
> > > > > > >> + * @see DEVICE_ATTACH()
> > > > > > >> + */
> > > > > > >> +METHOD void after_attach {
> > > > > > >> + device_t dev;
> > > > > > >> +} DEFAULT null_after_attach;
> > > > > > >> +
> > > > > > >> +/**
> > > > > > >> * @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
> > > > > > >> @@ -2743,6 +2743,7 @@ device_attach(device_t dev)
> > > > > > >> dev->state =3D DS_ATTACHED;
> > > > > > >> dev->flags &=3D ~DF_DONENOMATCH;
> > > > > > >> devadded(dev);
> > > > > > >> + DEVICE_AFTER_ATTACH(dev);
> > > > > > >> return (0);
> > > > > > >> }
> > > > > > >>=20
> > > > > > >=20
> > > > > > > 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 a=
cquired in
> > > > > > > the driver's cdev implementation routines before using any ne=
wbus
> > > > > > > functions other than device_get_softc(), would solve the prob=
lem without
> > > > > > > a driver api change that would make it harder to backport/MFC=
driver
> > > > > > > changes.
> > > > > >=20
> > > > > > Also, more generally, don't create the dev nodes before you are=
ready to=20
> > > > > deal with requests. Why do we need to uglify everything here? I=
f you can't=20
> > > > > do that, you can check a bit in the softc and return EBUSY or ENX=
IO on open if=20
> > > > > that bit says that your driver isn't ready to accept requests.
> > > > >=20
> > > > > I agree, this dosen't actually fix anything as the decision for w=
hat to put
> > > > > in your foo_attach() method rather than foo_after_attach() is non=
-obvious and=20
> > > > > very arbitrary.
> > > > >=20
> > > > > The actual bug appears to only be with using 'device_busy()'. I t=
hink
> > > > > 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 att=
ached
> > > > > which will not require any changes to drivers.
> > > >=20
> > > > Could you, please, elaborate your proposal ? How do you think devic=
e_busy()
> > > > can be enchanced ?
> > > >=20
> > > > Obvious idea to sleep inside device_busy() until dev->state becomes=
!=3D
> > > > 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.
> > >=20
> > > I think you could have a DS_ATTACHING state and allow device_busy() t=
o work
> > > for DS_ATTACHING. The idea being that it is a bug for a driver to in=
voke
> > > 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 w=
hen 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.
> >=20
> > Now, the driver needs to ensure that cdesvw->d_open() on dri/card0
> > would return ENXIO until dri/forcewake0 is created. This can be impleme=
nted
> > with flag, indeed. But still somewhat muddy, and probably leads to
> > user-visible errors (I mostly worry about graphical login managers).
>=20
> You could also sleep on the flag in d_open() (you can imagine a two-step
> process where you set a "adding cdev's flag", then all d_open() calls blo=
ck
> on it). Then when finished adding cdev's, you set a flag if an error
> occurred and wake up all the waiters. If no error occured the waiters can
> have the first open() work fine. But even with other proposals you still
> have to deal with this problem if you want to fail out entirely if you
> have problems creating cdevs.
>=20
> > But for single-node drivers it is indeed a nice solution.
>=20
> I think we are somewhat stuck with this for other reasons as well. Note
> that device_busy() propagates up the tree to parent devices, so imagine
> kldloading a driver that creates a tree (e.g. a bus with a few consumers)
> where the leaf devices are attached by a call to bus_generic_attach() from
> the device_attach() method for the parent device. Even if you add a
> DEVICE_AFTER_ATTACH() hook, while it may allow device_busy() to be invoked
> on the leaf device, when it tries to propagate device_busy() up to the
> parent device it would still be in the middle of attach and blow up anywa=
y.
This looks like a bug in my implementation of after_attach, which
apparently should be called for new tree after the top level attach
finished.
Anyway, would you commit your change ? I definitely can work out the
driver change after. But this seems to be a large amount of work for
driver authors.
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