Re: INFO: rcu detected stall in shrink_dcache_parent

[syzbot]

syzbot has found reproducer for 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=8e4a81166025b5b7fa66

So far this crash happened 13 times on upstream.
C reproducer: https://syzkaller.appspot.com/x/repro.c?id=5369092034789376
syzkaller reproducer:  
https://syzkaller.appspot.com/x/repro.syz?id=4909037686620160
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=6422767771582464
Kernel config:  
https://syzkaller.appspot.com/x/.config?id=-5947642240294114534
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+8e4a81166025b5b7fa66@xxxxxxxxxxxxxxxxxxxxxxxxx
It will help syzbot understand when the bug is fixed.

INFO: rcu_sched self-detected stall on CPU
	1-...!: (124998 ticks this GP) idle=c6e/1/4611686018427387906  
softirq=11062/11062 fqs=0
	 (t=125000 jiffies g=5220 c=5219 q=10)
rcu_sched kthread starved for 125000 jiffies! g5220 c5219 f0x0  
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
NMI backtrace for cpu 1
CPU: 1 PID: 4547 Comm: syzkaller613030 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:__preempt_count_add arch/x86/include/asm/preempt.h:76 [inline]
RIP: 0010:__rcu_read_lock include/linux/rcupdate.h:83 [inline]
RIP: 0010:rcu_read_lock include/linux/rcupdate.h:630 [inline]
RIP: 0010:d_walk+0x476/0xc80 fs/dcache.c:1303
RSP: 0018:ffff8801a93d79d8 EFLAGS: 00000203 ORIG_RAX: ffffffffffffff13
RAX: ffff8801acbe6300 RBX: ffff8801a762b520 RCX: ffffffff815cbe3e
RDX: 0000000000000000 RSI: ffffffff81c2a68f RDI: ffff8801a7644d20
RBP: ffff8801a93d7b58 R08: ffffed0034ec89a5 R09: ffffed0034ec89a4
R10: ffffed0034ec89a4 R11: ffff8801a7644d23 R12: ffff8801a7644ca0
R13: dffffc0000000000 R14: ffff8801a7644d20 R15: ffff8801a93d7b30
 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
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x4464a7
RSP: 002b:00007ffdd93b0718 EFLAGS: 00000207 ORIG_RAX: 0000000000000054
RAX: ffffffffffffffda RBX: 0000000000000065 RCX: 00000000004464a7
RDX: 0000000000000000 RSI: 00000000006d85f8 RDI: 00007ffdd93b1f80
RBP: 00007ffdd93b1f80 R08: 0000000000000000 R09: 0000000000000001
R10: 0000000000000005 R11: 0000000000000207 R12: 00000000025778a0
R13: 0000000000000000 R14: 000000000008d71f R15: 00007ffdd93b1978