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Subjecttty crash in Linux 4.6
Hi

In the kernel 4.6 I get crashes in the tty layer. I can reproduce the
crash by logging into the machine with ssh and typing before the prompt
appears.

The crash is caused by the pointer tty->disc_data being NULL in the
function n_tty_receive_buf_common. The crash happens on the statement
smp_load_acquire(&ldata->read_tail).

Bisecting shows that the crashes are caused by the patch
892d1fa7eaaed9d3c04954cb140c34ebc3393932 ("tty: Destroy ldisc instance on
hangup").

Kernel Fault: Code=15 regs=000000007d9e0720 (Addr=0000000000002260)
CPU: 0 PID: 3319 Comm: kworker/u8:0 Not tainted 4.6.0 #1
Workqueue: events_unbound flush_to_ldisc
task: 000000007c25ea80 ti: 000000007d9e0000 task.ti: 000000007d9e0000

YZrvWESTHLNXBCVMcbcbcbcbOGFRQPDI
PSW: 00001000000001000000000000001111 Not tainted
r00-03 000000000804000f 000000004076cd10 0000000040475fb4 000000007f761800
r04-07 0000000040749510 0000000000000001 000000007f761800 000000007d9e0490
r08-11 000000007e722890 0000000000000000 000000007da4ec00 000000007f763823
r12-15 0000000000000000 000000007fc08ea8 000000007fc08c78 000000004080e080
r16-19 000000007fc08c00 0000000000000001 0000000000000000 0000000000002260
r20-23 000000007f7618b0 000000007c25ea80 0000000000000001 0000000000000001
r24-27 0000000000000000 000000000800000f 000000007f7618ac 0000000040749510
r28-31 0000000000000001 000000007d9e0840 000000007d9e0720 0000000000000001
sr00-03 00000000086c8800 0000000000000000 0000000000000000 00000000086c8800
sr04-07 0000000000000000 0000000000000000 0000000000000000 0000000000000000

IASQ: 0000000000000000 0000000000000000 IAOQ: 0000000040475fd4 0000000040475fd8
IIR: 0e6c00d5 ISR: 0000000000000000 IOR: 0000000000002260
CPU: 0 CR30: 000000007d9e0000 CR31: ff87e7ffbc9ffffe
ORIG_R28: 000000004080a180
IAOQ[0]: n_tty_receive_buf_common+0xb4/0xbe0
IAOQ[1]: n_tty_receive_buf_common+0xb8/0xbe0
RP(r2): n_tty_receive_buf_common+0x94/0xbe0
Backtrace:
[<0000000040476b14>] n_tty_receive_buf2+0x14/0x20
[<000000004047a208>] tty_ldisc_receive_buf+0x30/0x90
[<000000004047a544>] flush_to_ldisc+0x144/0x1c8
[<00000000402556bc>] process_one_work+0x1b4/0x460
[<0000000040255bbc>] worker_thread+0x1e4/0x5e0
[<000000004025d454>] kthread+0x134/0x168

Mikulas

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