WARNING in percpu_ref_kill_and_confirm

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    9e5de623 Merge tag 'nfs-for-5.1-5' of git://git.linux-nfs...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15624257200000
kernel config:  https://syzkaller.appspot.com/x/.config?x=856fc6d0fbbeede9
dashboard link: https://syzkaller.appspot.com/bug?extid=10d25e23199614b7721f
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
userspace arch: i386
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17ff39f3200000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15758647200000

The bug was bisected to:

commit 38e7571c07be01f9f19b355a9306a4e3d5cb0f5b
Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date:   Fri Mar 8 22:48:40 2019 +0000

    Merge tag 'io_uring-2019-03-06' of git://git.kernel.dk/linux-block

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1736bc57200000
final crash:    https://syzkaller.appspot.com/x/report.txt?x=14b6bc57200000
console output: https://syzkaller.appspot.com/x/log.txt?x=10b6bc57200000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+10d25e23199614b7721f@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 38e7571c07be ("Merge tag 'io_uring-2019-03-06' of  
git://git.kernel.dk/linux-block")

------------[ cut here ]------------
percpu_ref_kill_and_confirm called more than once on io_ring_ctx_ref_free!
WARNING: CPU: 1 PID: 7815 at lib/percpu-refcount.c:335  
percpu_ref_kill_and_confirm+0x341/0x3b0 lib/percpu-refcount.c:335
Kernel panic - not syncing: panic_on_warn set ...
CPU: 1 PID: 7815 Comm: syz-executor269 Not tainted 5.1.0-rc5+ #77
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+0x172/0x1f0 lib/dump_stack.c:113
 panic+0x2cb/0x65c kernel/panic.c:214
 __warn.cold+0x20/0x45 kernel/panic.c:571
 report_bug+0x263/0x2b0 lib/bug.c:186
 fixup_bug arch/x86/kernel/traps.c:179 [inline]
 fixup_bug arch/x86/kernel/traps.c:174 [inline]
 do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:272
 do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:291
 invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:973
RIP: 0010:percpu_ref_kill_and_confirm+0x341/0x3b0 lib/percpu-refcount.c:335
Code: 42 e0 2a 06 01 48 89 fa 48 c1 ea 03 80 3c 02 00 75 76 49 8b 54 24 10  
48 c7 c6 a0 71 a1 87 48 c7 c7 40 71 a1 87 e8 ad 92 13 fe <0f> 0b 48 b8 00  
00 00 00 00 fc ff df 4c 89 ea 48 c1 ea 03 80 3c 02
RSP: 0018:ffff8880a96cfce0 EFLAGS: 00010086
RAX: 0000000000000000 RBX: 0000607f5142e35b RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff815afcf6 RDI: ffffed10152d9f8e
RBP: ffff8880a96cfd10 R08: ffff8880a85c40c0 R09: fffffbfff1133639
R10: fffffbfff1133638 R11: ffffffff8899b1c3 R12: ffff88809ee571c0
R13: ffff88809ee571c8 R14: 0000000000000286 R15: 0000000000000000
 percpu_ref_kill include/linux/percpu-refcount.h:128 [inline]
 __io_uring_register+0xa7/0x1fe0 fs/io_uring.c:2937
 __do_sys_io_uring_register fs/io_uring.c:2998 [inline]
 __se_sys_io_uring_register fs/io_uring.c:2980 [inline]
 __ia32_sys_io_uring_register+0x193/0x1f0 fs/io_uring.c:2980
 do_syscall_32_irqs_on arch/x86/entry/common.c:326 [inline]
 do_fast_syscall_32+0x281/0xc98 arch/x86/entry/common.c:397
 entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
RIP: 0023:0xf7f16869
Code: 85 d2 74 02 89 0a 5b 5d c3 8b 04 24 c3 8b 14 24 c3 8b 3c 24 c3 90 90  
90 90 90 90 90 90 90 90 90 90 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90  
90 90 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90
RSP: 002b:00000000f7ef11ec EFLAGS: 00000296 ORIG_RAX: 00000000000001ab
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
Kernel Offset: disabled
Rebooting in 86400 seconds..


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