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BUG: KCSAN: data-race in udpv6_sendmsg / udpv6_sendmsg

write to 0xffff888136d5fdf0 of 4 bytes by task 7819 on cpu 0:
 udpv6_sendmsg+0x15bc/0x16b0 net/ipv6/udp.c:1548
 inet6_sendmsg+0x5f/0x80 net/ipv6/af_inet6.c:642
 sock_sendmsg_nosec net/socket.c:654 [inline]
 sock_sendmsg net/socket.c:674 [inline]
 ____sys_sendmsg+0x360/0x4d0 net/socket.c:2337
 ___sys_sendmsg net/socket.c:2391 [inline]
 __sys_sendmmsg+0x315/0x4b0 net/socket.c:2477
 __do_sys_sendmmsg net/socket.c:2506 [inline]
 __se_sys_sendmmsg net/socket.c:2503 [inline]
 __x64_sys_sendmmsg+0x53/0x60 net/socket.c:2503
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff888136d5fdf0 of 4 bytes by task 7821 on cpu 1:
 udpv6_sendmsg+0x201/0x16b0 net/ipv6/udp.c:1328
 inet6_sendmsg+0x5f/0x80 net/ipv6/af_inet6.c:642
 sock_sendmsg_nosec net/socket.c:654 [inline]
 sock_sendmsg net/socket.c:674 [inline]
 ____sys_sendmsg+0x360/0x4d0 net/socket.c:2337
 ___sys_sendmsg net/socket.c:2391 [inline]
 __sys_sendmmsg+0x315/0x4b0 net/socket.c:2477
 __do_sys_sendmmsg net/socket.c:2506 [inline]
 __se_sys_sendmmsg net/socket.c:2503 [inline]
 __x64_sys_sendmmsg+0x53/0x60 net/socket.c:2503
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x00000000 -> 0x0000000a

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 7821 Comm: syz-executor.5 Not tainted 5.13.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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