On Mi, 2019-04-17 at 09:19 +0000, Kloetzke Jan wrote:
When disconnecting cdc_ncm the kernel sporadically crashes shortly
after the disconnect:
[ 57.868812] Unable to handle kernel NULL pointer dereference at virtual address 00000000
...
[ 58.006653] PC is at 0x0
[ 58.009202] LR is at call_timer_fn+0xec/0x1b4
[ 58.013567] pc : [<0000000000000000>] lr : [<ffffff80080f5130>] pstate: 00000145
[ 58.020976] sp : ffffff8008003da0
[ 58.024295] x29: ffffff8008003da0 x28: 0000000000000001
[ 58.029618] x27: 000000000000000a x26: 0000000000000100
[ 58.034941] x25: 0000000000000000 x24: ffffff8008003e68
[ 58.040263] x23: 0000000000000000 x22: 0000000000000000
[ 58.045587] x21: 0000000000000000 x20: ffffffc68fac1808
[ 58.050910] x19: 0000000000000100 x18: 0000000000000000
[ 58.056232] x17: 0000007f885aff8c x16: 0000007f883a9f10
[ 58.061556] x15: 0000000000000001 x14: 000000000000006e
[ 58.066878] x13: 0000000000000000 x12: 00000000000000ba
[ 58.072201] x11: ffffffc69ff1db30 x10: 0000000000000020
[ 58.077524] x9 : 8000100008001000 x8 : 0000000000000001
[ 58.082847] x7 : 0000000000000800 x6 : ffffff8008003e70
[ 58.088169] x5 : ffffffc69ff17a28 x4 : 00000000ffff138b
[ 58.093492] x3 : 0000000000000000 x2 : 0000000000000000
[ 58.098814] x1 : 0000000000000000 x0 : 0000000000000000
...
[ 58.205800] [< (null)>] (null)
[ 58.210521] [<ffffff80080f5298>] expire_timers+0xa0/0x14c
[ 58.215937] [<ffffff80080f542c>] run_timer_softirq+0xe8/0x128
[ 58.221702] [<ffffff8008081120>] __do_softirq+0x298/0x348
[ 58.227118] [<ffffff80080a6304>] irq_exit+0x74/0xbc
[ 58.232009] [<ffffff80080e17dc>] __handle_domain_irq+0x78/0xac
[ 58.237857] [<ffffff8008080cf4>] gic_handle_irq+0x80/0xac
...
The crash happens roughly 125..130ms after the disconnect. This
correlates with the 'delay' timer that is started on certain USB tx/rx
errors in the URB completion handler.
The suspected problem is a race of usbnet_stop() with
usbnet_start_xmit(). In usbnet_stop() we call usbnet_terminate_urbs()
to cancel all URBs in flight. This only makes sense if no new URBs are
submitted concurrently, though. But the usbnet_start_xmit() can run at
the same time on another CPU which almost unconditionally submits an
URB. The error callback of the new URB will then schedule the timer
after it was already stopped.
Hi,
interesting. How sure are you of the details of your analysis?
I am asking because usbnet_stop() does a del_timer_sync().
It is indeed written under the assumption that the upper layer
will have ceased transmission when it stops an interface.
So I am wondering whether the correct fix would not be to make
sure the timer is started.
Regards
Oliver