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There may be a race condition if f_fs calls unregister_gadget_item in ffs_closed() when unregister_gadget is called by UDC store at the same time. this leads to a kernel NULL pointer dereference: [ 310.644928] Unable to handle kernel NULL pointer dereference at virtual address 00000004 [ 310.645053] init: Service 'adbd' is being killed... [ 310.658938] pgd = c9528000 [ 310.662515] [00000004] *pgd=19451831, *pte=00000000, *ppte=00000000 [ 310.669702] Internal error: Oops: 817 [#1] PREEMPT SMP ARM [ 310.675211] Modules linked in: [ 310.678294] CPU: 0 PID: 1537 Comm: ->transport Not tainted 4.1.15-03725-g793404c #2 [ 310.685958] Hardware name: Freescale i.MX6 Quad/DualLite (Device Tree) [ 310.692493] task: c8e24200 ti: c945e000 task.ti: c945e000 [ 310.697911] PC is at usb_gadget_unregister_driver+0xb4/0xd0 [ 310.703502] LR is at __mutex_lock_slowpath+0x10c/0x16c [ 310.708648] pc : [<c075efc0>] lr : [<c0bfb0bc>] psr: 600f0113 <snip..> [ 311.565585] [<c075efc0>] (usb_gadget_unregister_driver) from [<c075e2b8>] (unregister_gadget_item+0x1c/0x34) [ 311.575426] [<c075e2b8>] (unregister_gadget_item) from [<c076fcc8>] (ffs_closed+0x8c/0x9c) [ 311.583702] [<c076fcc8>] (ffs_closed) from [<c07736b8>] (ffs_data_reset+0xc/0xa0) [ 311.591194] [<c07736b8>] (ffs_data_reset) from [<c07738ac>] (ffs_data_closed+0x90/0xd0) [ 311.599208] [<c07738ac>] (ffs_data_closed) from [<c07738f8>] (ffs_ep0_release+0xc/0x14) [ 311.607224] [<c07738f8>] (ffs_ep0_release) from [<c023e030>] (__fput+0x80/0x1d0) [ 311.614635] [<c023e030>] (__fput) from [<c014e688>] (task_work_run+0xb0/0xe8) [ 311.621788] [<c014e688>] (task_work_run) from [<c010afdc>] (do_work_pending+0x7c/0xa4) [ 311.629718] [<c010afdc>] (do_work_pending) from [<c010770c>] (work_pending+0xc/0x20) for functions using functionFS, i.e. android adbd will close /dev/usb-ffs/adb/ep0 when usb IO thread fails, but switch adb from on to off also triggers write "none" > UDC. These 2 operations both call unregister_gadget, which will lead to the panic above. add a mutex before calling unregister_gadget for api used in f_fs. Signed-off-by: Winter Wang <wente.wang@nxp.com> Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
To understand all the Linux-USB framework, you'll use these resources:
* This source code. This is necessarily an evolving work, and
includes kerneldoc that should help you get a current overview.
("make pdfdocs", and then look at "usb.pdf" for host side and
"gadget.pdf" for peripheral side.) Also, Documentation/usb has
more information.
* The USB 2.0 specification (from www.usb.org), with supplements
such as those for USB OTG and the various device classes.
The USB specification has a good overview chapter, and USB
peripherals conform to the widely known "Chapter 9".
* Chip specifications for USB controllers. Examples include
host controllers (on PCs, servers, and more); peripheral
controllers (in devices with Linux firmware, like printers or
cell phones); and hard-wired peripherals like Ethernet adapters.
* Specifications for other protocols implemented by USB peripheral
functions. Some are vendor-specific; others are vendor-neutral
but just standardized outside of the www.usb.org team.
Here is a list of what each subdirectory here is, and what is contained in
them.
core/ - This is for the core USB host code, including the
usbfs files and the hub class driver ("hub_wq").
host/ - This is for USB host controller drivers. This
includes UHCI, OHCI, EHCI, and others that might
be used with more specialized "embedded" systems.
gadget/ - This is for USB peripheral controller drivers and
the various gadget drivers which talk to them.
Individual USB driver directories. A new driver should be added to the
first subdirectory in the list below that it fits into.
image/ - This is for still image drivers, like scanners or
digital cameras.
../input/ - This is for any driver that uses the input subsystem,
like keyboard, mice, touchscreens, tablets, etc.
../media/ - This is for multimedia drivers, like video cameras,
radios, and any other drivers that talk to the v4l
subsystem.
../net/ - This is for network drivers.
serial/ - This is for USB to serial drivers.
storage/ - This is for USB mass-storage drivers.
class/ - This is for all USB device drivers that do not fit
into any of the above categories, and work for a range
of USB Class specified devices.
misc/ - This is for all USB device drivers that do not fit
into any of the above categories.