syzbot has found reproducer for the following crash on upstream commit
fd40ffc72e2f74c7db61e400903e7d50a88bc0b0 (Mon Apr 9 18:36:05 2018 +0000)
selinux: fix missing dput() before selinuxfs unmount
syzbot dashboard link: https://syzkaller.appspot.com/bug?extid=7e3f31a52646f939c052


So far this crash happened 731 times on upstream.
C reproducer: https://syzkaller.appspot.com/x/repro.c?id=5746654556717056
syzkaller reproducer: https://syzkaller.appspot.com/x/repro.syz?id=5437811310198784 Raw console output: https://syzkaller.appspot.com/x/log.txt?id=5066563195502592 Kernel config: https://syzkaller.appspot.com/x/.config?id=-771321277174894814
compiler: gcc (GCC) 8.0.1 20180301 (experimental)

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+7e3f31a52646f939c...@syzkaller.appspotmail.com
It will help syzbot understand when the bug is fixed.

INFO: rcu_sched self-detected stall on CPU
0-....: (124999 ticks this GP) idle=1f2/1/4611686018427387906 softirq=10492/10492 fqs=31246
         (t=125000 jiffies g=4909 c=4908 q=3)
NMI backtrace for cpu 0
CPU: 0 PID: 4481 Comm: syzkaller419220 Not tainted 4.16.0+ #14
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 <IRQ>
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1b9/0x294 lib/dump_stack.c:113
 nmi_cpu_backtrace.cold.4+0x19/0xce lib/nmi_backtrace.c:103
 nmi_trigger_cpumask_backtrace+0x151/0x192 lib/nmi_backtrace.c:62
 arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:38
 trigger_single_cpu_backtrace include/linux/nmi.h:156 [inline]
 rcu_dump_cpu_stacks+0x175/0x1c2 kernel/rcu/tree.c:1376
 print_cpu_stall kernel/rcu/tree.c:1525 [inline]
 check_cpu_stall.isra.61.cold.80+0x36c/0x59a kernel/rcu/tree.c:1593
 __rcu_pending kernel/rcu/tree.c:3356 [inline]
 rcu_pending kernel/rcu/tree.c:3401 [inline]
 rcu_check_callbacks+0x21b/0xad0 kernel/rcu/tree.c:2763
 update_process_times+0x2d/0x70 kernel/time/timer.c:1636
 tick_sched_handle+0xa0/0x180 kernel/time/tick-sched.c:171
 tick_sched_timer+0x42/0x130 kernel/time/tick-sched.c:1179
 __run_hrtimer kernel/time/hrtimer.c:1337 [inline]
 __hrtimer_run_queues+0x3e3/0x10a0 kernel/time/hrtimer.c:1399
 hrtimer_interrupt+0x286/0x650 kernel/time/hrtimer.c:1457
 local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1025 [inline]
 smp_apic_timer_interrupt+0x15d/0x710 arch/x86/kernel/apic/apic.c:1050
 apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:862
 </IRQ>
RIP: 0010:arch_local_irq_enable arch/x86/include/asm/paravirt.h:793 [inline]
RIP: 0010:snd_pcm_stream_unlock_irq+0xb7/0xf0 sound/core/pcm_native.c:166
RSP: 0018:ffff8801ac8a6b60 EFLAGS: 00000293 ORIG_RAX: ffffffffffffff13
RAX: ffff8801a89e2700 RBX: 0000000000000000 RCX: 1ffff1003513c5f3
RDX: 0000000000000000 RSI: ffffffff85a14e10 RDI: ffffffff88b17bf8
RBP: ffff8801ac8a6b68 R08: ffff8801a89e2f78 R09: 0000000000000006
R10: ffff8801a89e2700 R11: 0000000000000000 R12: 0000000000000004
R13: 00000000ffffffe0 R14: ffff8801d9a6ac00 R15: ffff8801ceb2f200
 __snd_pcm_lib_xfer+0x739/0x1d10 sound/core/pcm_lib.c:2246
 snd_pcm_oss_write3+0xe9/0x220 sound/core/oss/pcm_oss.c:1236
 snd_pcm_oss_write2+0x34c/0x460 sound/core/oss/pcm_oss.c:1373
 snd_pcm_oss_sync1+0x332/0x5a0 sound/core/oss/pcm_oss.c:1606
 snd_pcm_oss_sync.isra.29+0x790/0x980 sound/core/oss/pcm_oss.c:1682
 snd_pcm_oss_release+0x214/0x290 sound/core/oss/pcm_oss.c:2559
 __fput+0x34d/0x890 fs/file_table.c:209
 ____fput+0x15/0x20 fs/file_table.c:243
 task_work_run+0x1e4/0x290 kernel/task_work.c:113
 exit_task_work include/linux/task_work.h:22 [inline]
 do_exit+0x1aee/0x2730 kernel/exit.c:865
 do_group_exit+0x16f/0x430 kernel/exit.c:968
 get_signal+0x886/0x1960 kernel/signal.c:2469
 do_signal+0x98/0x2040 arch/x86/kernel/signal.c:810
 exit_to_usermode_loop+0x28a/0x310 arch/x86/entry/common.c:162
 prepare_exit_to_usermode arch/x86/entry/common.c:196 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:265 [inline]
 do_syscall_64+0x792/0x9d0 arch/x86/entry/common.c:292
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x449eb9
RSP: 002b:00007f302d286da8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: fffffffffffffe00 RBX: 00000000006dcc5c RCX: 0000000000449eb9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00000000006dcc5c
RBP: 00000000006dcc58 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0100000000000000
R13: 7073642f7665642f R14: 646e732f7665642f R15: 0000000000000009

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