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BUG: KCSAN: data-race in ipv6_setsockopt / udpv6_recvmsg

write to 0xffff88811ec39f6c of 2 bytes by task 612 on cpu 1:
 do_ipv6_setsockopt net/ipv6/ipv6_sockglue.c:649 [inline]
 ipv6_setsockopt+0x1b22/0x3710 net/ipv6/ipv6_sockglue.c:1003
 udpv6_setsockopt+0x85/0x90 net/ipv6/udp.c:1626
 sock_common_setsockopt+0x5d/0x70 net/core/sock.c:3247
 __sys_setsockopt+0x2dc/0x450 net/socket.c:2132
 __do_sys_setsockopt net/socket.c:2143 [inline]
 __se_sys_setsockopt net/socket.c:2140 [inline]
 __x64_sys_setsockopt+0x62/0x70 net/socket.c:2140
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88811ec39f6c of 2 bytes by task 608 on cpu 0:
 udpv6_recvmsg+0xa44/0xc50 net/ipv6/udp.c:421
 inet6_recvmsg+0xa3/0x210 net/ipv6/af_inet6.c:654
 ____sys_recvmsg+0x15a/0x2e0 include/linux/uio.h:234
 ___sys_recvmsg net/socket.c:2618 [inline]
 do_recvmmsg+0x35c/0xa30 net/socket.c:2716
 __sys_recvmmsg net/socket.c:2795 [inline]
 __do_sys_recvmmsg net/socket.c:2818 [inline]
 __se_sys_recvmmsg net/socket.c:2811 [inline]
 __x64_sys_recvmmsg+0xcc/0x150 net/socket.c:2811
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 608 Comm: syz-executor.5 Not tainted 5.8.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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