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BUG: KCSAN: data-race in ____sys_sendmsg / ipv6_setsockopt

write to 0xffff8880bb48aca0 of 8 bytes by task 10425 on cpu 1:
 do_ipv6_setsockopt net/ipv6/ipv6_sockglue.c:491 [inline]
 ipv6_setsockopt+0x35af/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 0xffff8880bb48aca0 of 8 bytes by task 10424 on cpu 0:
 sock_sendmsg_nosec net/socket.c:651 [inline]
 sock_sendmsg net/socket.c:671 [inline]
 ____sys_sendmsg+0x335/0x4d0 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: 0 PID: 10424 Comm: syz-executor.4 Not tainted 5.9.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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