INFO: rcu detected stall in vfs_rmdir

[syzbot]

Hello,

syzbot hit the following crash on upstream commit
16e205cf42da1f497b10a4a24f563e6c0d574eec (Fri Apr 13 03:56:10 2018 +0000)
Merge tag 'drm-fixes-for-v4.17-rc1' of  
git://people.freedesktop.org/~airlied/linux
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=da69f0eccf07cdcf6710

Unfortunately, I don't have any reproducer for this crash yet.
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=5006146695856128
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+da69f0eccf07cdcf6710@xxxxxxxxxxxxxxxxxxxxxxxxx
It will help syzbot understand when the bug is fixed. See footer for  
details.
If you forward the report, please keep this part and the footer.

INFO: rcu_sched self-detected stall on CPU
	0-...!: (124998 ticks this GP) idle=10e/1/4611686018427387906  
softirq=30640/30640 fqs=8
	 (t=125000 jiffies g=15921 c=15920 q=62)
rcu_sched kthread starved for 124967 jiffies! g15921 c15920 f0x0  
RCU_GP_WAIT_FQS(3) ->state=0x0 ->cpu=1
RCU grace-period kthread stack dump:
rcu_sched       R  running task    23768     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
NMI backtrace for cpu 0
CPU: 0 PID: 4595 Comm: syz-executor3 Not tainted 4.16.0+ #2
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+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
 </IRQ>
RIP: 0010:shrink_dcache_parent+0x174/0x230 fs/dcache.c:1484
RSP: 0018:ffff88018cacfb68 EFLAGS: 00000246 ORIG_RAX: ffffffffffffff13
RAX: 0000000000000000 RBX: dffffc0000000000 RCX: 0000000000000000
RDX: ffffffff81c20160 RSI: ffff88018cacfbf0 RDI: ffff8801d5252b80
RBP: ffff88018cacfc58 R08: ffff88018cac2480 R09: ffffed003aa4a580
R10: ffffed003aa4a580 R11: ffff8801d5252c03 R12: ffff88018cacfc30
R13: ffff88018cacfbf0 R14: 1ffff10031959f7e R15: ffffed0031959f81
 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
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x455087
RSP: 002b:00007fffa68fc118 EFLAGS: 00000206 ORIG_RAX: 0000000000000054
RAX: ffffffffffffffda RBX: 0000000000000065 RCX: 0000000000455087
RDX: 0000000000000000 RSI: 00007fffa68fdec0 RDI: 00007fffa68fdec0
RBP: 00007fffa68fdec0 R08: 0000000000000000 R09: 0000000000000001
R10: 000000000000000a R11: 0000000000000206 R12: 0000000001ea2940
R13: 0000000000000000 R14: 00000000000001b8 R15: 0000000000017faa


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