WARNING in reuseport_add_sock

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 8e170a58
git tree: android-4.9
console output: https://syzkaller.appspot.com/x/log.txt?x=143dba59800000
kernel config: https://syzkaller.appspot.com/x/.config?x=4fadd453521adb
dashboard link: https://syzkaller.appspot.com/bug?extid=23680fa2fc52e9626e9e
compiler: gcc (GCC) 7.1.1 20170620
userspace arch: i386
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=14463659800000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=14c2f1a9800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+23680f...@syzkaller.appspotmail.com

------------[ cut here ]------------
WARNING: CPU: 0 PID: 3327 at net/core/sock_reuseport.c:118
reuseport_add_sock+0x54b/0x650 net/core/sock_reuseport.c:116
socket already in reuseport group[ 19.711187] Kernel panic - not syncing:
panic_on_warn set ...

CPU: 0 PID: 3327 Comm: syzkaller221554 Not tainted 4.9.76-g8e170a5 #21
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
ffff8801cd53fa68 ffffffff81d93149 ffffffff83a47180 ffff8801cd53fb40
ffffffff83eea700 ffffffff82f9bbeb 0000000000000009 ffff8801cd53fb30
ffffffff8142e371 0000000041b58ab3 ffffffff84189678 ffffffff8142e1b5
Call Trace:
[<ffffffff81d93149>] __dump_stack lib/dump_stack.c:15 [inline]
[<ffffffff81d93149>] dump_stack+0xc1/0x128 lib/dump_stack.c:51
[<ffffffff8142e371>] panic+0x1bc/0x3a8 kernel/panic.c:179
[<ffffffff811300f4>] __warn+0x1c4/0x1e0 kernel/panic.c:542
[<ffffffff811301d4>] warn_slowpath_fmt+0xc4/0x110 kernel/panic.c:565
[<ffffffff82f9bbeb>] reuseport_add_sock+0x54b/0x650
net/core/sock_reuseport.c:116
[<ffffffff83215ce0>] inet_reuseport_add_sock
net/ipv4/inet_hashtables.c:455 [inline]
[<ffffffff83215ce0>] __inet_hash+0x5c0/0x900 net/ipv4/inet_hashtables.c:479
[<ffffffff83216088>] inet_hash+0x68/0xa0 net/ipv4/inet_hashtables.c:503
[<ffffffff8321cd19>] inet_csk_listen_start+0x2b9/0x360
net/ipv4/inet_connection_sock.c:757
[<ffffffff832e5fbc>] inet_listen+0x12c/0x2e0 net/ipv4/af_inet.c:241
[<ffffffff82ed8dd8>] SYSC_listen net/socket.c:1425 [inline]
[<ffffffff82ed8dd8>] SyS_listen+0x188/0x260 net/socket.c:1411
[<ffffffff81006fc7>] do_syscall_32_irqs_on arch/x86/entry/common.c:322
[inline]
[<ffffffff81006fc7>] do_fast_syscall_32+0x2f7/0x890
arch/x86/entry/common.c:384
[<ffffffff838b2334>] entry_SYSENTER_compat+0x74/0x83
arch/x86/entry/entry_64_compat.S:127
Dumping ftrace buffer:
(ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


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