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INFO: rcu detected stall in shrink_dcache_parent


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=8e4a81166025b5b7fa66

So far this crash happened 5 times on upstream.
Unfortunately, I don't have any reproducer for this crash yet.
Raw console output: https://syzkaller.appspot.com/x/log.txt?id=5775899525906432 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+8e4a81166025b5b7fa66@xxxxxxxxxxxxxxxxxxxxxxxxx
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TCP: request_sock_TCPv6: Possible SYN flooding on port 20002. Sending cookies. Check SNMP counters.
INFO: rcu_sched detected stalls on CPUs/tasks:
	(detected by 0, t=125007 jiffies, g=42063, c=42062, q=674)
All QSes seen, last rcu_sched kthread activity 125014 (4295035404-4294910390), jiffies_till_next_fqs=3, root ->qsmask 0x0
syz-executor7   R  running task    20696  4548   4539 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:arch_local_irq_restore arch/x86/include/asm/paravirt.h:783 [inline]
RIP: 0010:seqcount_lockdep_reader_access include/linux/seqlock.h:83 [inline]
RIP: 0010:read_seqcount_begin include/linux/seqlock.h:164 [inline]
RIP: 0010:read_seqbegin include/linux/seqlock.h:433 [inline]
RIP: 0010:read_seqbegin_or_lock include/linux/seqlock.h:529 [inline]
RIP: 0010:d_walk+0x840/0xc80 fs/dcache.c:1248
RSP: 0018:ffff88018cc0f9d8 EFLAGS: 00000293 ORIG_RAX: ffffffffffffff13
RAX: ffff88018cc02740 RBX: 0000000000000293 RCX: 1ffff100319805fd
RDX: 0000000000000000 RSI: ffffffff81c2a7ab RDI: 0000000000000293
RBP: ffff88018cc0fb58 R08: ffff88018cc02f78 R09: 0000000000000006
R10: ffff88018cc02740 R11: 0000000000000000 R12: 0000000000000200
R13: dffffc0000000000 R14: 1ffff10031981f7e R15: ffff88018cc0fb30
 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: 0033:0x454fe7
RSP: 002b:00007ffc1ddb5118 EFLAGS: 00000206 ORIG_RAX: 0000000000000054
RAX: ffffffffffffffda RBX: 0000000000000065 RCX: 0000000000454fe7
RDX: 0000000000000000 RSI: 00007ffc1ddb6ec0 RDI: 00007ffc1ddb6ec0
RBP: 00007ffc1ddb6ec0 R08: 0000000000000000 R09: 0000000000000001
R10: 000000000000000a R11: 0000000000000206 R12: 00000000011ab940
R13: 0000000000000000 R14: 00000000000004d0 R15: 000000000003b525
rcu_sched kthread starved for 125573 jiffies! g42063 c42062 f0x2 RCU_GP_WAIT_FQS(3) ->state=0x0 ->cpu=1
RCU grace-period kthread stack dump:
rcu_sched       R  running task    23592     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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