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SubjectRe: WARNING: refcount bug in io_wqe_worker
From
syzbot has found a reproducer for the following issue on:

HEAD commit: 729e3d09 Merge tag 'ceph-for-5.9-rc5' of git://github.com/..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12f2ce0d900000
kernel config: https://syzkaller.appspot.com/x/.config?x=c61610091f4ca8c4
dashboard link: https://syzkaller.appspot.com/bug?extid=956ef5eac18eadd0fb7f
compiler: gcc (GCC) 10.1.0-syz 20200507
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=10f3a853900000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=157f7263900000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+956ef5eac18eadd0fb7f@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 0 PID: 7382 at lib/refcount.c:28 refcount_warn_saturate+0x1d1/0x1e0 lib/refcount.c:28
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 7382 Comm: io_wqe_worker-1 Not tainted 5.9.0-rc4-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+0x198/0x1fd lib/dump_stack.c:118
panic+0x347/0x7c0 kernel/panic.c:231
__warn.cold+0x20/0x46 kernel/panic.c:600
report_bug+0x1bd/0x210 lib/bug.c:198
handle_bug+0x38/0x90 arch/x86/kernel/traps.c:234
exc_invalid_op+0x14/0x40 arch/x86/kernel/traps.c:254
asm_exc_invalid_op+0x12/0x20 arch/x86/include/asm/idtentry.h:536
RIP: 0010:refcount_warn_saturate+0x1d1/0x1e0 lib/refcount.c:28
Code: e9 db fe ff ff 48 89 df e8 2c dd 18 fe e9 8a fe ff ff e8 c2 d6 d8 fd 48 c7 c7 60 dc 93 88 c6 05 0d 0d 12 07 01 e8 b1 d7 a9 fd <0f> 0b e9 af fe ff ff 0f 1f 84 00 00 00 00 00 41 56 41 55 41 54 55
RSP: 0018:ffffc9000810fe08 EFLAGS: 00010286
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffff88809592a200 RSI: ffffffff815db9a7 RDI: fffff52001021fb3
RBP: 0000000000000003 R08: 0000000000000001 R09: ffff8880ae6318e7
R10: 0000000000000000 R11: 0000000000000000 R12: ffff88809a6080b8
R13: ffff8880a7896e30 R14: ffff8880a7896e00 R15: ffff8880a6727f00
refcount_sub_and_test include/linux/refcount.h:274 [inline]
refcount_dec_and_test include/linux/refcount.h:294 [inline]
io_worker_exit fs/io-wq.c:236 [inline]
io_wqe_worker+0xcdb/0x10e0 fs/io-wq.c:596
kthread+0x3b5/0x4a0 kernel/kthread.c:292
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294
Kernel Offset: disabled
Rebooting in 86400 seconds..

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