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SubjectINFO: rcu detected stall in do_signal (4)
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Hello,

syzbot found the following crash on:

HEAD commit: 2187f215 Merge tag 'for-5.5-rc2-tag' of git://git.kernel.o..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10031a8ee00000
kernel config: https://syzkaller.appspot.com/x/.config?x=ab2ae0615387ef78
dashboard link: https://syzkaller.appspot.com/bug?extid=7e600afa7f6059f9a30e
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=163852fee00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=135b43b6e00000

Bisection is inconclusive: the bug happens on the oldest tested release.

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=123dc849e00000
final crash: https://syzkaller.appspot.com/x/report.txt?x=113dc849e00000
console output: https://syzkaller.appspot.com/x/log.txt?x=163dc849e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+7e600afa7f6059f9a30e@syzkaller.appspotmail.com

hrtimer: interrupt took 92494 ns
rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
(detected by 0, t=10502 jiffies, g=8669, q=2)
rcu: All QSes seen, last rcu_preempt kthread activity 10502
(4295016466-4295005964), jiffies_till_next_fqs=1, root ->qsmask 0x0
syz-executor765 R running task 28552 9407 9389 0x00000000
Call Trace:
<IRQ>
sched_show_task kernel/sched/core.c:5954 [inline]
sched_show_task.cold+0x2ee/0x35d kernel/sched/core.c:5929
print_other_cpu_stall kernel/rcu/tree_stall.h:410 [inline]
check_cpu_stall kernel/rcu/tree_stall.h:538 [inline]
rcu_pending kernel/rcu/tree.c:2827 [inline]
rcu_sched_clock_irq.cold+0xaf4/0xc02 kernel/rcu/tree.c:2271
update_process_times+0x2d/0x70 kernel/time/timer.c:1726
tick_sched_handle+0xa2/0x190 kernel/time/tick-sched.c:167
tick_sched_timer+0x53/0x140 kernel/time/tick-sched.c:1310
__run_hrtimer kernel/time/hrtimer.c:1517 [inline]
__hrtimer_run_queues+0x364/0xe40 kernel/time/hrtimer.c:1579
hrtimer_interrupt+0x314/0x770 kernel/time/hrtimer.c:1641
local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1110 [inline]
smp_apic_timer_interrupt+0x160/0x610 arch/x86/kernel/apic/apic.c:1135
apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:829
</IRQ>
RIP: 0010:__raw_spin_unlock_irq include/linux/spinlock_api_smp.h:169
[inline]
RIP: 0010:_raw_spin_unlock_irq+0x4f/0x80 kernel/locking/spinlock.c:199
Code: c0 e8 34 93 89 48 ba 00 00 00 00 00 fc ff df 48 c1 e8 03 80 3c 10 00
75 33 48 83 3d 52 f5 ca 01 00 74 20 fb 66 0f 1f 44 00 00 <bf> 01 00 00 00
e8 e7 7b 88 f9 65 8b 05 d8 de 39 78 85 c0 74 06 41
RSP: 0000:ffffc90001f47ca0 EFLAGS: 00000282 ORIG_RAX: ffffffffffffff13
RAX: 1ffffffff132669d RBX: 0000000000000000 RCX: 0000000000000000
RDX: dffffc0000000000 RSI: 0000000000000006 RDI: ffff888095ca8b54
RBP: ffffc90001f47ca8 R08: ffff888095ca82c0 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffff8880a8ef0b80
R13: ffff8880a8ef0cc0 R14: dffffc0000000000 R15: 000000000000000b
spin_unlock_irq include/linux/spinlock.h:388 [inline]
get_signal+0x1bee/0x24f0 kernel/signal.c:2737
do_signal+0x87/0x1700 arch/x86/kernel/signal.c:815
exit_to_usermode_loop+0x286/0x380 arch/x86/entry/common.c:160
prepare_exit_to_usermode+0x321/0x3a0 arch/x86/entry/common.c:195
ret_from_intr+0x26/0x36
RIP: 0033:0x401505
Code: 00 00 00 be b4 fe 4a 00 e8 48 ee ff ff 48 85 c0 0f 85 d2 fe ff ff e9
4a ff ff ff e8 55 d9 04 00 48 6b 44 24 28 18 8b 7c 24 38 <48> 8b 88 50 00
00 20 48 8b 90 48 00 00 20 48 8b b0 40 00 00 20 e8
RSP: 002b:00007f51c6df1b20 EFLAGS: 00010202
RAX: 0000000002f54358 RBX: 00000000006dcc28 RCX: 0000000000406cc7
RDX: 60b237aef639e3c2 RSI: 0000000000000000 RDI: 0000000000000004
RBP: 00000000006dcc20 R08: 00007f51c6df1b20 R09: 00000000006dcc20
R10: 0000000000000000 R11: 0000000000000000 R12: 00000000006dcc2c
R13: 00007ffcd25bbb6f R14: 00007f51c6df29c0 R15: 00000000006dcc20
rcu: rcu_preempt kthread starved for 10502 jiffies! g8669 f0x2
RCU_GP_WAIT_FQS(5) ->state=0x0 ->cpu=1
rcu: RCU grace-period kthread stack dump:
rcu_preempt R running task 29272 10 2 0x80004000
Call Trace:
context_switch kernel/sched/core.c:3385 [inline]
__schedule+0x934/0x1f90 kernel/sched/core.c:4081
schedule+0xdc/0x2b0 kernel/sched/core.c:4155
schedule_timeout+0x486/0xc50 kernel/time/timer.c:1895
rcu_gp_fqs_loop kernel/rcu/tree.c:1661 [inline]
rcu_gp_kthread+0x9b2/0x18d0 kernel/rcu/tree.c:1821
kthread+0x361/0x430 kernel/kthread.c:255
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
For information about bisection process see: https://goo.gl/tpsmEJ#bisection
syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches

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