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BUG: KCSAN: data-race in __ip6_datagram_connect / udpv6_sendmsg

write to 0xffff8881217f6040 of 4 bytes by task 11741 on cpu 0:
 __ipv6_addr_set_half include/net/ipv6.h:560 [inline]
 ipv6_addr_set include/net/ipv6.h:569 [inline]
 ipv6_addr_set_v4mapped include/net/ipv6.h:737 [inline]
 __ip6_datagram_connect+0x1a0/0x900 net/ipv6/datagram.c:197
 ip6_datagram_connect+0x2a/0x40 net/ipv6/datagram.c:272
 inet_dgram_connect+0x115/0x200 net/ipv4/af_inet.c:577
 __sys_connect_file net/socket.c:1852 [inline]
 __sys_connect+0x245/0x280 net/socket.c:1869
 __do_sys_connect net/socket.c:1879 [inline]
 __se_sys_connect net/socket.c:1876 [inline]
 __x64_sys_connect+0x3d/0x50 net/socket.c:1876
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8881217f6040 of 4 bytes by task 11770 on cpu 1:
 ipv6_addr_v4mapped include/net/ipv6.h:695 [inline]
 udpv6_sendmsg+0x864/0x1780 net/ipv6/udp.c:1334
 inet6_sendmsg+0x5f/0x80 net/ipv6/af_inet6.c:638
 sock_sendmsg_nosec net/socket.c:651 [inline]
 sock_sendmsg net/socket.c:671 [inline]
 ____sys_sendmsg+0x360/0x4d0 net/socket.c:2353
 ___sys_sendmsg net/socket.c:2407 [inline]
 __sys_sendmmsg+0x322/0x4b0 net/socket.c:2497
 __do_sys_sendmmsg net/socket.c:2526 [inline]
 __se_sys_sendmmsg net/socket.c:2523 [inline]
 __x64_sys_sendmmsg+0x53/0x60 net/socket.c:2523
 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: 1 PID: 11770 Comm: syz-executor.3 Not tainted 5.9.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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