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SubjectWARNING in percpu_ref_exit
From
Hello,

syzbot found the following crash on:

HEAD commit: 7ddd09fc Add linux-next specific files for 20191220
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1457dcb9e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=f183b01c3088afc6
dashboard link: https://syzkaller.appspot.com/bug?extid=2eea1ab51194c814cb70
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=116182c1e00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16d3c925e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+2eea1ab51194c814cb70@syzkaller.appspotmail.com

------------[ cut here ]------------
WARNING: CPU: 1 PID: 9727 at lib/percpu-refcount.c:111
percpu_ref_exit+0xab/0xd0 lib/percpu-refcount.c:111
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 9727 Comm: syz-executor571 Not tainted
5.5.0-rc2-next-20191220-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x197/0x210 lib/dump_stack.c:118
panic+0x2e3/0x75c kernel/panic.c:221
__warn.cold+0x2f/0x3e kernel/panic.c:582
report_bug+0x289/0x300 lib/bug.c:195
fixup_bug arch/x86/kernel/traps.c:174 [inline]
fixup_bug arch/x86/kernel/traps.c:169 [inline]
do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:267
do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:286
invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1027
RIP: 0010:percpu_ref_exit+0xab/0xd0 lib/percpu-refcount.c:111
Code: 00 00 00 00 fc ff df 48 c1 ea 03 80 3c 02 00 75 1d 48 c7 43 08 03 00
00 00 e8 01 41 e5 fd 5b 41 5c 41 5d 5d c3 e8 f5 40 e5 fd <0f> 0b eb bf 4c
89 ef e8 29 2c 23 fe eb d9 e8 82 2b 23 fe eb a7 4c
RSP: 0018:ffffc90003bf7968 EFLAGS: 00010293
RAX: ffff8880a700e500 RBX: ffff8880990cde10 RCX: ffffffff83901432
RDX: 0000000000000000 RSI: ffffffff8390149b RDI: ffff8880990cde28
RBP: ffffc90003bf7980 R08: ffff8880a700e500 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000607f514357c0
R13: ffff8880990cde18 R14: ffff8880973e3000 R15: ffff8880973e3228
io_sqe_files_unregister+0x7d/0x2f0 fs/io_uring.c:4623
io_ring_ctx_free fs/io_uring.c:5575 [inline]
io_ring_ctx_wait_and_kill+0x430/0x9a0 fs/io_uring.c:5644
io_uring_release+0x42/0x50 fs/io_uring.c:5652
__fput+0x2ff/0x890 fs/file_table.c:280
____fput+0x16/0x20 fs/file_table.c:313
task_work_run+0x145/0x1c0 kernel/task_work.c:113
exit_task_work include/linux/task_work.h:22 [inline]
do_exit+0x909/0x2f20 kernel/exit.c:797
do_group_exit+0x135/0x360 kernel/exit.c:895
get_signal+0x47c/0x24f0 kernel/signal.c:2734
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 arch/x86/entry/common.c:195 [inline]
syscall_return_slowpath arch/x86/entry/common.c:278 [inline]
do_syscall_64+0x676/0x790 arch/x86/entry/common.c:304
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x4468f9
Code: e8 0c e8 ff ff 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
ff 0f 83 5b 07 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fb197749db8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: fffffffffffffe00 RBX: 00000000006dbc48 RCX: 00000000004468f9
RDX: 0000000000000000 RSI: 0000000000000080 RDI: 00000000006dbc48
RBP: 00000000006dbc40 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006dbc4c
R13: 00007ffc42ed8b0f R14: 00007fb19774a9c0 R15: 0000000000000001
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
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:
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syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches

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