On Thu, 13 Nov 2008, Jiri Kosina wrote:
> On Wed, 12 Nov 2008, Alan Stern wrote:
>
> > [ 795.103175] =======================================================
> > [ 795.103347] [ INFO: possible circular locking dependency detected ]
> > [ 795.103439] 2.6.28-rc4 #1
> > [ 795.103522] -------------------------------------------------------
> > [ 795.103614] rmmod/2553 is trying to acquire lock:
> > [ 795.103703] (&dev->mutex){--..}, at: [<c054680e>] input_unregister_device+0x16/0x15d
> > [ 795.103957]
> > [ 795.103958] but task is already holding lock:
> > [ 795.104060] (&usbhid->setup){--..}, at: [<f077a69e>] usbhid_stop+0x40/0xf8 [usbhid]
> > [ 795.104060] which lock already depends on the new lock.
>
> Hmm, I don't think I really understand the root cause here on a first
> sight. We always take usbhid->setup mutex in usbhid_stop() and after that
> call into input_unregister_device(), which acquires the corresponding
> input_dev->mutex.
>
> But I don't seem to be able find where we take input_dev->mutex before
> taking usbhid->setup mutex ... I somehow don't see that neither from the
> lockdep traces nor the code.
Here's what the lockdep trace indicates:
Somewhere in the input_register_device / input_attach_handler
path, you lock input_dev->mutex. While that lock is held,
kbd_connect calls down to usbhid_open, which calls
usb_autopm_get_interface, which acquires usb_dev->pm_mutex.
hid_suspend is called with usb_dev->pm_mutex held. It
acquires usbhid->setup.
Therefore we end up with the following 3-way loop:
The unregister path acquires input_dev->mutex while holding
usbhid->setup.
The register/attach/open path acquires usb_dev->pm_mutex
while holding input_dev->mutex.
The suspend path acquires usbhid->setup while holding
usb_dev->pm_mutex.
I'm not familiar enough with the usbhid subsystem to know the right way
to break this loop.
Alan Stern
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