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INFO: rcu detected stall in d_walk


syzbot hit the following crash on upstream commit
c17b0aadb7d8f87de56a4a374a8131519c0f7422 (Thu Apr 12 16:15:48 2018 +0000)
Merge tag 'asm-generic' of git://git.kernel.org/pub/scm/linux/kernel/git/arnd/asm-generic syzbot dashboard link: https://syzkaller.appspot.com/bug?extid=ae80b790eb412884ca77

So far this crash happened 3 times on upstream.
C reproducer: https://syzkaller.appspot.com/x/repro.c?id=6595600812343296
syzkaller reproducer: https://syzkaller.appspot.com/x/repro.syz?id=6204964946837504 Raw console output: https://syzkaller.appspot.com/x/log.txt?id=5012331448762368 Kernel config: https://syzkaller.appspot.com/x/.config?id=-5947642240294114534
compiler: gcc (GCC) 8.0.1 20180301 (experimental)

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Reported-by: syzbot+ae80b790eb412884ca77@xxxxxxxxxxxxxxxxxxxxxxxxx
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)1{��^f��D�.`*�e��B�8H8��O�" or missing value
EXT4-fs (sda1): Unrecognized mount option "�辭 m��.2�-[)3i�~�r�����$S�%j�?� ����ƅ�|��1���>�[���2 H�s�no[�n�߲$��;��g�
)1{��^f��D�.`*�e��B�8H8��O�" or missing value
INFO: rcu_sched detected stalls on CPUs/tasks:
	(detected by 1, t=125007 jiffies, g=5317, c=5316, q=46)
All QSes seen, last rcu_sched kthread activity 125016 (4295416981-4295291965), jiffies_till_next_fqs=3, root ->qsmask 0x0
syzkaller106625 R  running task    21976  4551   4549 0x00000008
Call Trace:
 sched_show_task.cold.87+0x27a/0x301 kernel/sched/core.c:5325
 print_other_cpu_stall.cold.79+0x92f/0x9d2 kernel/rcu/tree.c:1481
 check_cpu_stall.isra.61+0x706/0xf50 kernel/rcu/tree.c:1599
 __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+0x9f/0x180 kernel/time/tick-sched.c:173
 tick_sched_timer+0x45/0x130 kernel/time/tick-sched.c:1283
 __run_hrtimer kernel/time/hrtimer.c:1386 [inline]
 __hrtimer_run_queues+0x3e3/0x10a0 kernel/time/hrtimer.c:1448
 hrtimer_interrupt+0x286/0x650 kernel/time/hrtimer.c:1506
 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
RIP: 0010:list_empty include/linux/list.h:203 [inline]
RIP: 0010:select_collect+0x175/0x5f0 fs/dcache.c:1465
RSP: 0018:ffff8801c79e78e8 EFLAGS: 00000293 ORIG_RAX: ffffffffffffff13
RAX: ffff8801d95765c0 RBX: 1ffff10038f3cf20 RCX: ffffffff81c1ff94
RDX: 0000000000000000 RSI: ffffffff81c20025 RDI: ffff8801c79e7c08
RBP: ffff8801c79e79c8 R08: ffff8801d95765c0 R09: ffffed0034ffdec8
R10: ffffed0034ffdec8 R11: ffff8801a7fef643 R12: ffff8801c79e7bf0
R13: ffff8801a7fef5c0 R14: ffff8801c79e7bf8 R15: ffff8801c79e79a0
 d_walk+0x3c3/0xc80 fs/dcache.c:1276
 shrink_dcache_parent+0x179/0x230 fs/dcache.c:1486
 vfs_rmdir+0x202/0x470 fs/namei.c:3850
 do_rmdir+0x523/0x610 fs/namei.c:3911
 SYSC_rmdir fs/namei.c:3929 [inline]
 SyS_rmdir+0x1a/0x20 fs/namei.c:3927
 do_syscall_64+0x29e/0x9d0 arch/x86/entry/common.c:287
RIP: 0033:0x4423c7
RSP: 002b:00007ffe66511a38 EFLAGS: 00000207 ORIG_RAX: 0000000000000054
RAX: ffffffffffffffda RBX: 0000000000000065 RCX: 00000000004423c7
RDX: 0000000000000000 RSI: 00000000006d45f8 RDI: 00007ffe66513250
RBP: 00007ffe66513250 R08: 0000000000000000 R09: 0000000000000001
R10: 000000000000000a R11: 0000000000000207 R12: 0000000001a048a0
R13: 0000000000000000 R14: 000000000009880b R15: 00007ffe66512c48
rcu_sched kthread starved for 125589 jiffies! g5317 c5316 f0x2 RCU_GP_WAIT_FQS(3) ->state=0x0 ->cpu=0
RCU grace-period kthread stack dump:
rcu_sched       R  running task    23896     9      2 0x80000000
Call Trace:
 context_switch kernel/sched/core.c:2848 [inline]
 __schedule+0x801/0x1e30 kernel/sched/core.c:3490
 schedule+0xef/0x430 kernel/sched/core.c:3549
 schedule_timeout+0x138/0x240 kernel/time/timer.c:1801
 rcu_gp_kthread+0x6b5/0x1940 kernel/rcu/tree.c:2231
 kthread+0x345/0x410 kernel/kthread.c:238
 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:411

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