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BUG: KCSAN: data-race in inet6_csk_route_socket / ip6_sk_dst_store_flow

write to 0xffff8881214e94d0 of 8 bytes by task 13358 on cpu 0:
 ip6_dst_store include/net/ip6_route.h:236 [inline]
 inet6_csk_route_socket+0x408/0x420 net/ipv6/inet6_connection_sock.c:109
 inet6_csk_xmit+0x91/0x1f0 net/ipv6/inet6_connection_sock.c:121
 l2tp_xmit_core net/l2tp/l2tp_core.c:1030 [inline]
 l2tp_xmit_skb+0x8c9/0x8e0 net/l2tp/l2tp_core.c:1132
 pppol2tp_sendmsg+0x2fc/0x3c0 net/l2tp/l2tp_ppp.c:325
 sock_sendmsg_nosec net/socket.c:637 [inline]
 sock_sendmsg+0x9f/0xc0 net/socket.c:657
 ___sys_sendmsg+0x2b7/0x5d0 net/socket.c:2311
 __sys_sendmmsg+0x123/0x350 net/socket.c:2413
 __do_sys_sendmmsg net/socket.c:2442 [inline]
 __se_sys_sendmmsg net/socket.c:2439 [inline]
 __x64_sys_sendmmsg+0x64/0x80 net/socket.c:2439
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff8881214e94d0 of 8 bytes by task 13360 on cpu 1:
 ip6_dst_store include/net/ip6_route.h:236 [inline]
 ip6_sk_dst_store_flow+0x260/0x290 net/ipv6/route.c:2821
 ip6_sk_dst_lookup_flow net/ipv6/ip6_output.c:1190 [inline]
 ip6_sk_dst_lookup_flow+0x36c/0x530 net/ipv6/ip6_output.c:1178
 udpv6_sendmsg+0x11bb/0x1c20 net/ipv6/udp.c:1446
 inet6_sendmsg+0x6d/0x90 net/ipv6/af_inet6.c:576
 sock_sendmsg_nosec net/socket.c:637 [inline]
 sock_sendmsg+0x9f/0xc0 net/socket.c:657
 sock_write_iter+0x16b/0x210 net/socket.c:989
 call_write_iter include/linux/fs.h:1895 [inline]
 do_iter_readv_writev+0x487/0x5b0 fs/read_write.c:693
 do_iter_write fs/read_write.c:970 [inline]
 do_iter_write+0x13b/0x3c0 fs/read_write.c:951
 vfs_writev+0x118/0x1c0 fs/read_write.c:1015
 do_writev+0xe3/0x250 fs/read_write.c:1058
 __do_sys_writev fs/read_write.c:1131 [inline]
 __se_sys_writev fs/read_write.c:1128 [inline]
 __x64_sys_writev+0x4e/0x60 fs/read_write.c:1128
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 13360 Comm: syz-executor.1 Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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