Re: 2.6.27-rc5-mmotm0829 - CONFIG_HID_COMPAT causes hangs at boot

[Valdis.Kletnieks@vt.edu]

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(Adding Alan Cox to the cc: in case he can shed light on this one - it
appears that HID_COMPAT only puts the bullet in the chamber, and doesn't
actually cause the hang...)

On Tue, 02 Sep 2008 23:02:42 +0200, Jiri Slaby said:
> On 09/02/2008 06:11 PM, Valdis.Kletnieks@vt.edu wrote:
> > The following 3 lines don't go to console by default due to loglevel setting.
> > So I'm not sure exactly where it hangs. But it's somewhere in here.

Right around here, we kick off a modprobe for 'hid_dummy'.
> >>> [    1.959193] usbcore: registered new interface driver usbhid
> >>> [    1.973037] usbhid: v2.6:USB HID core driver

And since my initrd doesn't include any modules (since until now, I've had
a kernel that can everything builtin so it can boot far enough to do the
whole udev/modprobe off my root filesystem, and hid_dummy is a new one on
me), this modprobe spits out a:

modprobe: FATAL: Could not open '/lib/modules/2.6.27-rc5-mmotm0829/modules.dep': No such file or directory

Well, yeah.. No modules on the initrd, so no modules.dep.  But having spewed
its error message, modprobe apparently decides to go off in a snit and hang.
Eventually, the usermode_helper call does a wait() on the modprobe, and then
*that* hangs because modprobe isn't returning.  And eventually the whole
level of initcalls comes to a screeching halt...

And here's the totally unexpected kernel traceback for the modprobe:

schedule_timeout+0x22/0xb4
? _raw_spin_lock+0xce/0x186
? _raw_spin_unlock+0xb7/0xe0
wait_for_common+-xb2/0xfb
? default_wake_function+0x0/0xf
wait_for_completion+0x18/0x1a
flush_cpu_workqueue+0x6b/0x77
? wq_barrior_func+0x0/0xf
flush_workqueue+0x4f/0x68
flush_scheduled_work+0x10/0x12
tty_ldisc_release+0x4a/0x21e
? _raw_pin_lock+0xce/0x186
? debug_mutex_unlock+0x127/0x14d
? mutex_unlock_slowpath+0x14a/0x15c
tty_release_dev+0x4da/0x508
? get_parent_ip+0x11/0x41
? get_parent_ip+0x11/0x41
tty_release+0x19/0x24
__fput+0xd9/0x198
fput+0x15/0x17
filp_close+0x67/0x72
sys_close+0xa9/0x104
system_call_fastpath+0x16/0x1b

WTF? We hang trying to close a tty??!?

So HID_COMPAT is basically off the hook here - it did the 'modprobe hid-dummy'
which *should* have just caused modprobe to print a nasty message and we
keep going... The *real* problem is that the modprobe then wedged up trying
to close the file descriptor after printing the nasty message...

Odd thought - is this modprobe popping before we've managed to fully set
up userspace I/O to the console?  It certainly *seems* to be firing before
the nash init script on the initrd starts running (the script starts off:

#!/bin/nash
mount -t proc /proc /proc
setquiet
echo Mounting proc filesystem

And we never see that 'echo'. So the modprobe is happening in *really* early
userspace...

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