Under stress occasions some TI devices might not return early when
reading the status register during the quirk invocation of xhci_irq made
by usb_hcd_pci_remove.  This means that instead of returning, we end up
handling this interruption in the middle of a shutdown.  Since
xhci->event_ring has already been freed in xhci_mem_cleanup, we end up
accessing freed memory, causing the Oops below.

commit 8c24d6d7 ("usb: xhci: stop everything on the first call to
xhci_stop") is the one that changed the instant in which we clean up the
event queue when stopping a device.  Before, we didn't call
xhci_mem_cleanup at the first time xhci_stop is executed (for the shared
HCD), instead, we only did it after the invocation for the primary HCD,
much later at the removal path.  The code flow for this oops looks like
this:

xhci_pci_remove()
	usb_remove_hcd(xhci->shared)
	        xhci_stop(xhci->shared)
 			xhci_halt()
			xhci_mem_cleanup(xhci);  // Free the event_queue
	usb_hcd_pci_remove(primary)
		xhci_irq()  // Access the event_queue if STS_EINT is set. Crash.
		xhci_stop()
			xhci_halt()
			// return early

The fix modifies xhci_stop to only cleanup the xhci data when releasing
the primary HCD.  This way, we still have the event_queue configured
when invoking xhci_irq.  We still halt the device on the first call to
xhci_stop, though.

I could reproduce this issue several times on the mainline kernel by
doing a bind-unbind stress test with a specific storage gadget attached.
I also ran the same test over-night with my patch applied and didn't
observe the issue anymore.

[  113.334124] Unable to handle kernel paging request for data at address 0x00000028
[  113.335514] Faulting instruction address: 0xd00000000d4f767c
[  113.336839] Oops: Kernel access of bad area, sig: 11 [#1]
[  113.338214] SMP NR_CPUS=1024 NUMA PowerNV

[c000000efe47ba90] c000000000720850 usb_hcd_irq+0x50/0x80
[c000000efe47bac0] c00000000073d328 usb_hcd_pci_remove+0x68/0x1f0
[c000000efe47bb00] d00000000daf0128 xhci_pci_remove+0x78/0xb0
[xhci_pci]
[c000000efe47bb30] c00000000055cf70 pci_device_remove+0x70/0x110
[c000000efe47bb70] c00000000061c6bc __device_release_driver+0xbc/0x190
[c000000efe47bba0] c00000000061c7d0 device_release_driver+0x40/0x70
[c000000efe47bbd0] c000000000619510 unbind_store+0x120/0x150
[c000000efe47bc20] c0000000006183c4 drv_attr_store+0x64/0xa0
[c000000efe47bc60] c00000000039f1d0 sysfs_kf_write+0x80/0xb0
[c000000efe47bca0] c00000000039e14c kernfs_fop_write+0x18c/0x1f0
[c000000efe47bcf0] c0000000002e962c __vfs_write+0x6c/0x190
[c000000efe47bd90] c0000000002eab40 vfs_write+0xc0/0x200
[c000000efe47bde0] c0000000002ec85c SyS_write+0x6c/0x110
[c000000efe47be30] c000000000009260 system_call+0x38/0x108
Signed-off-by: Gabriel Krisman Bertazi <krisman@linux.vnet.ibm.com>
Cc: Roger Quadros <rogerq@ti.com>
Cc: joel@jms.id.au
Cc: stable@vger.kernel.org
Reviewed-by: Roger Quadros <rogerq@ti.com>
Cc: <stable@vger.kernel.org> #v4.3+
Tested-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>