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BUG: KCSAN: data-race in __ip6_append_data / ipv6_setsockopt

write to 0xffff888073de9a2e of 2 bytes by task 26933 on cpu 0:
 do_ipv6_setsockopt net/ipv6/ipv6_sockglue.c:929 [inline]
 ipv6_setsockopt+0x16f2/0x3b80 net/ipv6/ipv6_sockglue.c:1003
 udpv6_setsockopt+0x76/0x80 net/ipv6/udp.c:1626
 sock_common_setsockopt+0x53/0x60 net/core/sock.c:3247
 __sys_setsockopt+0x322/0x4a0 net/socket.c:2138
 __do_sys_setsockopt net/socket.c:2149 [inline]
 __se_sys_setsockopt net/socket.c:2146 [inline]
 __x64_sys_setsockopt+0x62/0x70 net/socket.c:2146
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888073de9a2e of 2 bytes by task 26938 on cpu 1:
 ip6_sk_ignore_df include/net/ip6_route.h:282 [inline]
 __ip6_append_data+0x45d/0x2190 net/ipv6/ip6_output.c:1453
 ip6_make_skb+0x258/0x420 net/ipv6/ip6_output.c:1946
 udpv6_sendmsg+0x14f7/0x1780 net/ipv6/udp.c:1525
 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:2359
 ___sys_sendmsg net/socket.c:2413 [inline]
 __sys_sendmsg+0x1df/0x260 net/socket.c:2446
 __do_sys_sendmsg net/socket.c:2455 [inline]
 __se_sys_sendmsg net/socket.c:2453 [inline]
 __x64_sys_sendmsg+0x42/0x50 net/socket.c:2453
 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: 26938 Comm: syz-executor.1 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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